What does 美少年 Bishounen mean?

Bishounen means a beautiful boy. The word is descriptive, and it also names an aesthetic, a young man of androgynous beauty, with roots in older Japanese literature.

The characters

美 (bi) is beauty, the same character in words for fine art and for beautiful scenery. 少年 (shounen) is a boy, built from 少, few, and 年, year: a person of few years.

That second half is the same compound that names the shonen publishing demographic, which is a useful accident. It shows how ordinary the parts are: nothing in the word points at comics, romance, or a style of drawing.

The pattern is productive

Swap the second element and the counterpart appears, formed identically for a girl. Swap the first and other compounds emerge in the same shape.

What English does with it instead

Anglophone fandom uses it to name a character type: slender, delicate, long-lashed, styled to be attractive in a way that declines the usual signals of masculinity. That type genuinely exists in manga and has a long history in shoujo-adjacent art, so the usage points at something real.

The Japanese word covers both. One is a description available for any beautiful boy, including a perfectly conventional-looking one. The other is a specific aesthetic with a lineage.

Why English took up the word

English needed a name for the aesthetic and did not have one. The available options were unhelpful: pretty boy is faintly mocking, and androgynous is clinical and often inaccurate, since many of these characters are not ambiguous so much as beautiful under a different set of conventions.

So a word arrived attached to exactly the characters English wanted to name, and it got recruited for the job.

The register, which is the part usually missing

Applied to a real person it is a straightforward compliment, not a wink and not a genre reference, and it carries no implication about who the person is attracted to. English usage sometimes smuggles that implication in, and the Japanese word does not supply it.