{adj. phr.}, {informal} As tall as a very small child; very young. * /Charles started reading when he was knee-high to a grasshopper./ * /I've known Mary ever since she was knee-high to a duck./
KNEE-HIGH TO A DUCK
Meaning of KNEE-HIGH TO A DUCK in English
Dictionary of English idioms . Словарь английских идиом. 2012