this phrase is actually an acronym. its a combined acronym with another at the end. such as "rofl". people then started adding copter at the end. "soul" is added to the acronym "stfu". its origin came from a boy named Soul`1k on counter-strike. people didn't really like him so they all just said stfu soul. so now people commonly use it to make fool of someone when they are talking
Kelvin: hey, so did i tell you
Arnold: No you didn't now stfu soul.
Kelvin: LOL WTF