To make a mistake, to go wrong.

“You really fucked up this time.”

To botch or make a mess of.

“I fucked up question 3 of the exam.”

To injure or damage badly.

“I fucked up my knee while running the marathon.”

A serious mistake.

“Forgetting our anniversary was a major fuckup.”

One who continually makes mistakes.

“You've got to fire that fuckup.”

An ineffective person; a person who fucks up a lot

“Don't mind me, I'm just a harmless fuckup.”