= to command
"I said the word, ma'am," said Mr. Bumble. "The prerogative of a man is to command." - Mr. Bumble, Oliver Twist
Thus man's perogative = to command.
when a man changes his hairstyle. Usually switching from a comb-back to a comb-over.
Abby: I noticed you changed your hairstyle
Eric: What about it?
Abby: I like it
Eric: It's a man's perogative