Short for 'you know what to do'.
Mostly used at the end of a sentence to hint something without having to actually spell it out for the reader, whilst adding a solemn, dire, suggestive or perhaps a cheeky tone for either comical or epic purposes.
-During a zombie apocalypse-
Badass 1: u don't have to do this alone...
Badass 2: yes I do, u must take care of the rest. I won't be coming back. if the zombies break through the barrier..
Badass 2: YKWTD. *hands over grenade to badass 1*
Badass 1: gl u beast.
Badass 2: gg mate.
Dude 1: I heard ur little sister listening to Justin Bieber. YKWTD.
Dude 2: hahahah, let me just grab my shovel dude xD
eavesdropping little sister: *reads text* 0__0