a slip of a

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 now, 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 every 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
Farlex Dictionary of Idioms.

a slip of a —

a young, small, and slim person.
1980 Philip Larkin Letter After all you are a very young 51! Hardly 51 at all! A slip of a thing!
See also: of, slip
Farlex Partner Idioms Dictionary
See also:
  • a slip of a boy, girl, etc.
  • slip of a (something)
  • the slip
  • give (one) the slip
  • give somebody the slip
  • give someone the slip
  • give the slip
  • slip of the tongue
  • a slip of the tongue
  • slip (one) some skin