Fat End First - inserting the fat end of something into a bodily cavity. This is usually with the intent to cause pain. Anything with a fat end and a slender end can be used in this manner: carrots, baseball bats, and water bottles to name a few. Depending on the bodily cavity selected, this may result in difficulty in extracting the object. This acronym is generally used as a verb to describe the action of the penetration.
Joe just fef'd me with a carrot.
Go fef yourself with a baseball bat.
I just fef'd Caleb with a bowling pin.