1. A very attractive male, usually named Emmanuel, that can bring all the girls to the yard--without milkshake. Also looks incredible while applying hair product.
2. A sweetheart with a personality so charming it will give you diabetes, but no one's complaining. In fact, girls flock to him like they flock to muffins, hence the origin of the term. See above.
"Wow, He's such a stud muffin...He must be named Emmanuel".
--"Naw, they call him Manny for short."
This was actually coined in 1985 in Long Beach, California. It is actually the opposite of what most people think it is. It's more like muffin-top over your pants than a guy who is hot. It's a burn, not praise. It somehow got bastardized when it went viral.
A stud Muffin is someone who is a ripped dude at the gym who gets all the Pu$$y and gets laid like a chicken does eggs, 24/7. He is that one guy who has been liked throughout his whole school career and does more weight lifting and getting the ladies than actual homework, he is the STUD MUFFIN.
"Have you seen Brad lately? What a Stud Muffin, He is getting huge! Almost as big as his ego"