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Mokkori (モッコリ) is a Japanese sound symbol word (gitaigo) that essentially represents the "sound" of something bulging out, usually under cloth. It is probably related to another gitaigo "mukumuku" (ムクムク) which is the sound of something rising very quickly, towering. A popular term with fans of 1980s Japanese animation, it was popularized in the manga and anime City Hunter by Tsukasa Hojo. Ryo Saeba, the main character of City Hunter, took jobs to help people in need, always on the lookout for a "mokkori chance" (in the context of the series, a chance to do something perverted with an attractive woman, such as groping her buttocks). The term basically represents the sound or concept of a penile erection.
The slang term "schwing" is a close English equivalent.