Love or infatuation of very young persons; calf love(1834+)
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with puppy love
Also,calf love. Adolescent love or infatuation, especially one that is not expected to last, as in Beth is besotted with him, but we think it's just puppy love, or He's got a bad case of calf love.
[ First half of 1800s