1. Silly; crazy.
2. Completely absorbed, infatuated, or excited: They were gaga over the rock group's new album.
3. Senile; doddering.

[French, old fool, gaga, of imitative origin.]

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.