Originially orated by The United States of America's first President General George Washington as "I cannot tell a lie". This was later liberalized as "I'm not going to lie" then urbanized.
This statement is used to emphasize a statement in which you want to convey an earnest opinion without judgement.
"When my bookie cut off my finger and broke my legs with a bat it wasn't pleasant, i'm not gonna lie"
prtypr1nc3ss: omg did you see mark at the party? I wanted to mount him right there imngl!
sizeslutxoxo: lol imngl me2