a slip of a
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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."
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!