1. Either referring to muscle, a muscular man, or the act of building muscle.
2. A word referring to a vaginal fart. Otherwise known as a "queef".
"Tracy, whatchu doin', girl?"
"Building some mousa."
"Jenny, what was that noise?!"
"OMG, so fucking sorry. I just had to mousa."
"Oh, I always have to after sex, it's alright. Let your mousa out, bb."