Eye dialect spelling of "suppose", representing Southern US English. (Wikipedia has an arcticle on the subject "Eye dialect" for those who want to know more.)
I s'pose this is an acceptable explanation.
I s'pose two examples will be enough.
Nickname given to bales of marijuana thrown overboard or out of airplanes. Origin is reportedly from members of the US Coast Guard in the area of Southern Florida.
A sunburn or suntan focused on the left arm. It is caused by the left side of the body being the only half that gets exposed to sunlight while driving. Presumably, it would be located on the right side of the body in countries where they drive on the left side of the road.
I had a trucker's tan after driving for 10 straight hours on a sunny day.