The one week out of the year when a man has a cold, and doesn't feel good, and excercises his right to complain and whine- contrary to every fucking day of a woman's life when we have to hear you complain.
The man flu is usually accompanied by a lack of sympathy from females, which further advances the adverse symptoms of the man flu.
GF: Ugh, BF is so annoying and whiny, all he has is a cold.
BFF: Oh, yeah, the Man Flu.
BF: GOD FORBID you take care of ME for ONE DAY and not complain.