Commonly used to describe women that welcome men between their legs. One can tell that a woman constantly wants to fuck when she not only throws herself upon all those around her, but also make unnecessary sexual references to anything in the area.
To see if a woman is in SWTF mode, simply ask them if they like penis. If they respond with a smile, SWTF. If they don't, politely back away and apologize for the slip of the tongue.
Man #1: Wow, that bitch is really slutty. How many guys has she slept with?
Man #2: None. She is so horny I think she is about to hump that dude's leg. Imma go ask if SWTF.
My little sister's friend walks up to me and randomly gives me a hug. SWTF
Go to Taco Bell. The cashier gives me my change and she brushes my hand. SWTF