noun
partially exposed breasts; breasts bulging out of clothing
Slang term describing breasts that are not fully covered by clothing, often due to a low-cut top or loose garment. Used in casual, often male-oriented conversation.
彼女のドレスからはみ乳が見えそうだった。
Her dress almost revealed a bit of underboob.
あのグラビアアイドルのはみ乳がすごい。
That gravure idol's sideboob is amazing.
From はみ出る (to protrude) + 乳 (breast). The exact origin is uncertain, but it likely emerged in colloquial speech to describe the visual effect of breasts spilling out of clothing.