The act of two males grinding on a female, while the male on the frontside fingers the female. The behind male may do the same, creating a sandwich.
The two guys were sandwiching the girl on the dance floor, both fingering her, creating a finger sandwich.
The act of making one's self vomit after a meal, by using a finger (preferably middle) to gag and expel the contents of the stomach.
I followed up that hearty Thanksgiving Day dinner with a delicious finger sandwich. I feel so much thinner now!