Grammar and understanding of the concept is two different thing. It looks like that more than one people give you the pump up. If there are ten people, each people give you a thump up, then ten people is ten thumps up. I don't follow the grammatical rules, my understanding is more than one people give you the thump up.

well when we use the expression give it a big thumbs up, its usually because if its a very good thing or siutaion it will get 2 thumbs up, such as a film or movie. You hardly see it in its singular form. give it a thumb up. It's always been said to mean both thumbs.

A big thumBs-up. I think you should see "thumbs up" as one word.