a slip of a boy, girl, etc.

slip of a (something)

A very slip, petite person or thing. (Most often completed by "girl," "boy," or "thing.") You wouldn't think it to look at me know, but I was just a slip of a boy growing up. No one suspected that the tiny slip of a girl roaming the square was pickpocketing ever tourist she passed. A: "Wow, you got a short story published?" B: "It's just a slip of a thing, but I'm pretty pleased all the same."See also: of, slip

a ˈslip of a boy, girl, etc.

(old-fashioned) a small or thin, usually young, person: They were amazed that such a slip of a girl could cause so much trouble.See also: of, slip