noun
thin dōjinshi (often adult)
Manga/anime fandom slang for a self-published booklet, typically thin and often containing explicit content. The term plays on the literal meaning 'thin book' but is understood in context as a euphemism for adult dōjinshi.
See also: 同人誌
コミケで薄い本を買いあさる。
I go on a buying spree for thin dōjinshi at Comiket.
彼の鞄には薄い本が何冊も入っていた。
His bag had several thin dōjinshi in it.
同人誌 is the general term for self-published fan works, while 薄い本 specifically refers to thin, often adult-oriented dōjinshi in fandom slang.
From 薄い (thin) + 本 (book). The term arose in otaku circles as a euphemism for adult dōjinshi, which are typically thin booklets.