Flex
Flex has 2 meanings
Fantasy football position that can be a RB, WR, or TE
Flex is a fantasy football (FFB) term for a position that can be filled by a RB, WR, or TE. It is a popular lineup position used in many FFL formats.
Most FFLs feature starting lineups with 1 QB, 2 RBs, 3 WRs (or 2 WRs and the flex position), 1 TE, 1 K, and 1 team defense and special teams. The flex position is helpful because it doesn't limit FFB owners to only one position option when it comes to replacing injured players or players on bye weeks.
Example
I put my TE in at flex and he somehow won it for me by two points on Monday night
How well do you know your Fantasy Football slang?
A slang term for an action a person does to show off; derives from body builders "flexing" their muscles to show off in a competition; now used to describe a person who does or says something, often stupid, to impress others.