The words literally indicate a sexual let down.
But the spirit of the expression almost always points to someone reneging on a promise.
Someone presenting you with an attractive proposition but when time comes, they deliver you something you didn't really bargain for, is doing a TTNK with you.
Thullu Thorsi Nela Keyysu,
(Promise the glory-hole; Present the rat-hole.)
Thullu= cunt, Thorsi= after showing, Nela=ground, Keyysu=make you fuck.