僕 じゃない — Tae Kim's Japanese Grammar flashcard

[Who did this?!] Not me! -> In English, you'd rather use past tense (it wasn't me!). There are many differences like this in terms of grammar, it's something you need to get used to.

"boku janai" means "not me". The reason for "not me" again is given by context, e.g. someone may have asked, who cooks lunch today or who did something: not me.

boku: I/me (used by males)