Think With One's Little Head Meaning
(idiomatic, euphemistic, usually humorous, of a male) To make decisions or act based on one's sexual impulses rather than based on clear reasoning.
Example: 2003 May 9, Dalton Ross, "Rumble in the Jungle," Entertainment Weekly (retrieved 10 Jan 2010):
Obsessed with Heidi's cleavage, the pervert within may start thinking with his little head instead of his big one.
2007, Linnea Sinclair,The Down Home Zombie Blues, ISBN 9780553589641, p. 234:
[A]ll Theo could think about were all the places on her body he wanted to put his mouth. He was definitely, as Zeke was prone to say, thinking with his little head and not his big one.
2008 May 28, Antonia Zerbisias, "Bernier left ex-girlfriend twisting in media wind," Toronto Star (retrieved 10 Jan 2010):
[I]t's fair to say that now-disgraced former foreign affairs minister Maxime Bernier was thinking with his little headâ€”as well as his fat one.