Spit It is a new slang phrase with a meaning similar to "shut your face" or "shut the front door", which is commonly perceived as an angry and more commanding attempt to stop someone from talking, being stupid or making noise. More forceful forms of the phrase may be constructed by the interposition of modifiers, including "shut the hell up" and "shut the fuck up". In instant messenger communications, these are in turn often abbreviated to, SI, STHU and STFU, respectively.
If used incorrectly, or out of context....you will be labeled as a douchebag.
Jerkface: Baby, I'm so sorry for being such an a-hole to you...you deserve so much better and apparently I can't give it to you.
Baby: SPIT IT LOSER!