Where the frame comes from
The image is a frame from the Japanese anime series 'We Never Learn' (Japanese: 'Bokutachi wa Benkyou ga Dekinai,' abbreviated Bokuben), an adaptation of the manga of the same name published in Weekly Shonen Jump. The anime adaptation aired in April 2019. The series revolves around a high school student who tutors three classmates, each of whom excels in one subject while failing all the others catastrophically.
In one scene the camera shows a close-up of a woman's legs with a blue bra lying on the floor, and a subtitle appears at the bottom of the frame: 'Oh no, my bra fell off.' This is the English localization of the line, which plays on the comic awkwardness of the moment. It was this still frame with its caption that became a standalone meme, detached from the specific plot context of the episode.
How the frame became a template
Screenshots like this, with a close-up and a captioned line of dialogue, are a common format in anime meme communities. A frame is pulled from an episode and used as a reaction or as a template for a pun caption, often unrelated to the original scene. 'Bra fell off' took its place alongside other frames from the same series and related high-school anime, which spread as reactions across Twitter, Reddit, thematic VKontakte pages, and Pikabu (a Russian social news site).
The format proved convenient because of the contrast between a mundane awkward situation (a fallen piece of clothing) and the theatrically dramatic tone of the 'oh no' caption. That combination easily transfers to any occasion that prompts a light sense of embarrassment or shame.
How it is used
The meme is usually applied as a template for a playful 'confession' of one's own carelessness, blunder, or unlucky turn of events. The caption is reworked for the specific situation, keeping the recognizable structure 'oh no, ... fell/happened.' Typical variants include 'oh no, my deadline passed' or 'oh no, my phone fell into the toilet.' The original meaning of the frame is used as a form rather than as a literal statement about any particular piece of clothing.
Because the original scene is fan service in nature, the meme took hold mainly in anime communities and among the series' own viewers. In the wider internet it is known rather as one of many similar 'reaction' frames from Japanese animation, not as a phenomenon with a distinct history of its own.
Unlike many other viral frames, the 'fallen bra' has no single documented moment of becoming a standalone meme. The exact date of the first publication of this still frame apart from the episode, and the name of the user who first cut out and captioned it, are not known. It is therefore more accurate to speak of a gradual, diffuse spread within fan communities rather than a single viral explosion.
Its place in the wider context of the series
'We Never Learn' as a whole gave the internet several such reaction frames. Besides the 'fallen bra,' a format with an anxious heroine named Fumino from the ninth episode also spread, built on a similar four-panel meme principle. This suggests the series has been a convenient source of visual reactions for fan communities thanks to its expressive character faces and abundance of close-ups. It does not mean the bra frame was conceived by the creators as anything out of the ordinary. For the school harem comedy genre, such scenes are quite typical.