Two Thumbs Up

What does the phrase "a thumbs up" mean? I did some haphazard Googling, and always found it written "a thumbs up," rather than "a thumb's up." However, it seems like giving a thumbs-up means only one thumb is up, so it should be a thumb-up. My theory is it should actually be a thumb's up, using up like a noun as he "The basketball player has mad ups."