Be extremely enthusiastic; also, be extremely fond of. For example, Ellen is nuts about opera, or Kevin has been nuts about Megan since he met her. This seemingly new slangy expression began life in the late 1700s as nuts on or upon; about began to be substituted about 1940.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.