knee-high to a grasshopper
Quite young, as in I haven't seen him since I was knee-high to a grasshopper. This hyperbolic expression, dating from about 1850 and alluding to someone's youth, replaced the earlier knee-high to a mosquito or bumblebee or splinter.
Exploring A Runner’s High And Other MythsA runner's high seems pretty spectacular. All of your aches, pains, and emotional baggage just vanish after a kick-ass workout.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.