Misspent youth

[Raeanne Dimick]

Said of oneself by a (usually older) person when they demostrate in front of unsuspecting onlookers a physical skill that belies their innocent looks or background. Such a skill usually [reveals] experience with a [seedy], suspicious, questionable, or possibly illegal activty. Its a humorous self-deprecating [confession] of sorts that this skill was acquired by having practiced it in their youth; whether thats actually true or not.