SITFU, short for Suck It The Fuck Up. Commonly used by men towards whining women or other men that are whining. Used by military members towards other military members that are whining.
The Sgt told the Gunny that his head hurts from his old lady beating him with a baseball bat. The Gunny tells him to go to medical, and get some SITFU to make it better. The Sgt asks, "what's SITFU"? The Gunny says, "SUCK IT THE FUCK UP"!!!!!