Stands for 'What The Mother Fuck'. It first showed up on fazed.net when things were posted that even left regular internet users in disbelief, confusion, or shock. It is used when WTF (what the fuck) simply does not explain the confusion someone was left in.
WTmotherF a girl is trying to catch a banana flying out of someone's ass??
Anal banana launching etiquette? WTmotherF....