He definitely uses the word willowy. It's when he's describing how he got tricked at the Drones into falling into the pool (which he never really seems to get over!)

I found this quote from Thank You Jeeves:
But I don't look like old Glossop. We're built on different lines. Me – slender and willowy; him ... well, I don't wish to say anything derogatory concerning one who is bound to the aunt of an old friend by ties warmer than those of ... well, what I'm driving at is that you couldn't by any stretch of the imag. call him slender and willowy.