High School Make-Out Session, when two people make-out passionately, usually in public, long after they have graduated from high school. The make-out session tends to last too long and be too sloppy, which is totally unbecoming of two adults, but they simply haven't stepped up their game after all these years. The term also implies that the hook-up will not make it past a make-out, again, just like high school. The term carries a bit of shame with it because it shows a lack of bedroom prowess, but at the same time HSMOS's are fun because they remind you of being a kid and tend to be safer and less awkward afterward than say sex.
"How did things go with that guy at the bar last night?"
"Nothing, just an HSMOS, but he said he'd call"