Means whatever you want it to, but generally not used in a positive way. When used constantly it is one of the most ABNOXIOUS words in the english language. Southern in origin.
A person of marginal intelligence who enjoys partaking in the exploration of upward concavities. Contrary to popular belief, runt muffins are usually obese, despite their limited diet of female genitalia.
Schmandie is such a runtmuffin, did you see the way she looked at the cashier's nether regions?