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.
"I'm not gonna lie, but that's some stank cologne you're wearing".
"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