Combination of "whey" and "waifu".

Whey is a milk-based protein used by fitness people for building muscle.

Waifu is a term coined by otakus for their fictional anime/manga wives.

A wheyfu is a girl, real or not, who hits the gym for strength and you find attractive.

Similar to waifu but wheyfus are usually bodybuilders or really muscular/fit. The "whey" in wheyfu comes from the whey protein, which is usually used as a dietary supplement, popular among athletes after physical exercise to compensate the energy spent during the workout.

Derivative of waifu, a waifu that is ripped or very muscular.