To become wildly irrational; freak out, go ape: speculation that maybe old Strom had gone bananas at last
To be extremely enthusiastic; admire enormously: She went bananas over the dress and bought one in every color
[late 1960s+; fr the spectacle of an ape greedily gobbling bananas]
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 go bananas
Act crazy, as in When it comes to animal rights, some people go bananas. According to the lexicographer J. E. Lighter, this expression may allude to the similar go ape, in that apes and other primates are closely associated with eating bananas.
[ ; second half of 1900s