To secretly have sexual activities going on while eating at a bar or fancy restaurant under a table with the opposite sex it is anything that involves being sexual while trying to keep your composure
A term coined by user “mr. floof” on a YouTube comment, being under the table is when someone is out of the closet, but has to act straight in public when with their same sex partner. It’s usually due to being afraid that homophobic people might try to mess with them or be rude.
Girlfriend 1: “Babe, why won’t you hold my hand?”
Girlfriend 2: “Shh, I’m under the table right now! I think I just spotted a homophobe giving us a weird look.”