Either you have witnessed a public display of affection and wish to creating hilarity for all by yelling this glorified phrase or you wish to add a supreme amount of emphasis to anything and everything.
The phrase must be said with over the top enthusiasm.
A straight back and forth motion of the arm is also encouraged in attempts to hyper enforce the phrase.
Frank: Watch me drink these beers
You: GET SUM
Sandra: I'm going for a job interview
Derek: You'll do fine, as long as you don't forget to GET SUM(makes motion with arm)
Guy one to Guy two: "Yo look Pete is pulling that hot blonde outta here"
Guy one and Guy two yelling from across the bar: "GET SUUUUUM"
Leroy: I've got a date
Bobby: Two words
Leroy: um...GET SOME?
Bobby: You know it. Tropic Thunder style baby(makes glorified arm motion)
Wrongly confused by most people to be a saying for some1 who is getting sex.