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.
Guy A: My girlfriendbroke up with me today.
Guy B: Why?
Guy A: She said I acted too much like Edward Cullen.
Guy B: ...What, did you stalk her?
Guy A: ...Yeah..
Guy B: Dude, total FEF!