"Xanny" (stylized in all lowercase) is a song by American singer Billie Eilish. The song is the first promotional single from her debut studio album, When We All Fall Asleep, Where Do We Go? (2019). The singer and her brother Finneas O'Connell co-wrote it, whilst the latter handled its production. Musically, it was described as a mid-tempo and jazz-influenced pop ballad. Eilish recalled being inspired by several artists during the track's creation, most notably by Frank Sinatra. Productionwise, her distorted falsetto vocals are prominently layered over a similarly deformed bass in the drop of "Xanny", replicating the feeling of secondhand smoke. Lyrically, Eilish addresses substance abuse among teenagers; she was specifically inspired by the negative effects it had on her friends and ultimately on her.

Upon its release, "Xanny" received positive reviews from music critics, with one of them likening it to the works of Sophie and Lana Del Rey. The track was accompanied by a self-directed music video released on December 5, 2019. Minimalistically produced, it depicts the singer in an all-white outfit sitting on a bench against a white background whilst her face is burned by disembodied hands holding cigarette butts. Reviewers praised the visual's simplicity and its correlation to the song. With the release of its parent album, "Xanny" reached number 35 on the US Billboard Hot 100. Alongside top ten peaks in several countries, it was certified platinum in the United States by the Recording Industry Association of America (RIAA). Eilish included the track on the setlists of her 2019 When We All Fall Asleep Tour and 2020 Where Do We Go? World Tour.

"Xanny" is the third track on Eilish's debut studio album, When We All Fall Asleep, Where Do We Go?, released for digital download and streaming on March 29, 2019, by Darkroom and Interscope Records. The song was written by the singer and her brother Finneas O'Connell; the latter solely produced it. The duo has cited Frank Sinatra and Feist's "So Sorry" (2007) as primary influences during the creation process. They were also inspired by tracks such as LCD Soundsystem's "New York I Love You, But You’re Bringing Me Down" (2007), Post Malone's "Stay" (2018), Johnny Mathis's "Misty" (1959), Daniel Caesar's "Japanese Denim" (2016) and Melody Gardot's "Who Will Comfort Me" (2009). Mastering and mixing were handled by John Greenham and Rob Kinelski, respectively. Inspiration for "Xanny" came to Eilish after seeing friends heavily inebriating themselves at a party, consequently becoming "completely not who they were". The event had, in turn, a frustrating and affecting impact on the singer. Eilish said drugs was a close subject to her, since some of her friends had died due to drug overdose; the song's last verse, described as a "melancholy blow", had been recorded two days after one's death.

The title is a reference to the drug Alprazolam, sold under the brand name Xanax. It is the most prescribed benzodiazepine—medication for anxiety and panic disorders—in the United States. Side effects of the drug include fatigue, dizziness, memory impairment, hypersensitivity, irritability, and depression. Xanax is also highly addictive and heavily misused; it is to some extent responsible for around one-third of attempts at suicide by overdose. Hannah Rose Ewens of The Guardian felt that Xanax had been glamourised by young "disaffected" SoundCloud rappers.

Eilish elaborated that the message of "Xanny" was "less 'don't do drugs' [and] more 'be safe'". She endorses what Vox's Charlie Harding described as a "teen sobriety trend" in the lines: "I'm in their secondhand smoke / Still just drinking canned Coke / I don't need a Xanny to feel better". Other "self-aware" lyrics include: "Please don't try to kiss me on the sidewalk / On your cigarette break / I can't afford to love someone / Who isn't dying by mistake in Silver Lake".

Musically, "Xanny" runs at a moderately slow tempo of 54–56 beats per minute (BPM), and is played in the key of A major. Eilish's vocals, which are "near-whisper[ed]" throughout, span a range between the notes of F#₃ and E₅. Critical commentary described the song as a jazz-influenced mid-tempo pop ballad. While the "lulling" verses of "Xanny" utilize a jazz-inspired loop and a drumkit as instrumentation, Eilish's layered falsetto vocals are distorted for the track's "violent" drop in its refrain, and played over similarly deformed, "bone-rattling" bass. Staccato drum beats are also used throughout the song. Brendan Wetmore of Paper noted that the singer's vocals are manipulated in a way to "peak into an untapped vibrational territory".

Elaborating on the song's "weird" and unconventional sound, O'Connell told MTV that it was a point of contention with their label, although "it's kind of ironic, because [Eilish's] voice sounds so beautiful and the chords are really pretty". On the same occasion, he revealed that the "purposely distorted and clipped" sound of aforementioned SoundCloud artists influenced that of "Xanny". Eilish and O'Connell wanted the song's refrain to sound like "a girl blowing cigarette smoke into [someone's] face" and to replicate "what it feels like to be in secondhand smoke". Furthermore, their overall desire was to make people listening to the track "feel miserable".

Upon release, "Xanny" was generally praised by music critics. Insider's Libby Torres said that, although the track "lacks the slick hooks or sardonic lyrics of other songs on the album", Eilish's "breathy vocals and dismissive attitude towards recreational pill use make it one of the project's hidden gems". Pitchfork's Stacey Anderson commended the lyrical content, which, according to her, "wonderfully underscore[s] how all teen angst is both fiercely sincere and an effect of being only partially informed". Brendan Wetmore, writing for Paper magazine, stated that the "extremely jarring and tone-oscillating track" was influenced by the "modern pop" genre, which, as claimed by him, was created by the likes of Sophie and Lana Del Rey. Thomas Smith of NME stated that the musical experimentation on When We All Fall Asleep, Where Do We Go? was best represented by "Xanny". While likening the track's bridge to the works of the Beatles, Variety's Chris William saw the song as "tragicomic": "[I]t's anti-drug, in an amusing, WTF-is-wrong-with-my-contemporaries kind of way".