Copyright: male swimwear challenge
Art challenge where a female character is made to wear male swimwear while also topless and in a public space such as a beach or pool. Works best with an androgynous-looking short-haired female character with a boyish haircut and a flat chest or small breasts, but art featuring obviously female characters with longer hair and/or larger busts can also suffice, possibly for definite titillation of the viewer and embarrassment for the subject. Tomboys are frequently used as subjects to emphasize their masculine personalities.
While art of girls in male swimwear have been around for quite some time, the first direct response to the challenge appears to have been from November 21, 2020 on Twitter and Pixiv.
See also
This tag implicates swimwear (learn more).