IDKWTDWY means “I don’t know what to do without you” Which is a lyric from the song “Francis Forever” by Mitski. It’s a abbreviation from the first lyric of the song
Means i don't fucking know what the fuck. Can be used with "is this" or other suffixes. Used most when you're really shocked of what someone is showing to you and that you don't know what it is.
Guy 1 : "Man, do you know what is this horrible phallicobject?"
Guy 2 : "idfkwtf is this bro"