According to adults (generally parents of young children): small, hairy monkey that lives in the back of one's throat; also known as "throat troll". Commonly transmitted by young children expressing affection (i.e. kissing a.k.a. swappin' spit, tonsil hockey, etc.) to the opposite sex (or "crush").
Girl: "Ew, Susie got cooties from Timmy!"
Friend's response: "Again?!"