A person (who should be a female) who gave birth to you and in theese days is the center of all jokes and is used when the answer to something is very simple (maybe even writen above your head) but you want to include someones mom in some nerds are even offended by them
"Hey whatcha doin"
nerd: "hi what are yo doing"
bob: "yo momma!"
nerd: hey thats not nice my moms nice and wouln't sleep with you im telling"