ICWYDT is also commonly mistaken for ISWYDT
(which means 'I see what you did there') because of 'text language' and 'internet slang' using the letter 'C' as the word 'see.'
ICWYDT means "I 'see' what you did there."
'I see what you did there' is commonly used as a sarcastic response, as it is a clever turn of phrase.
Person 1: Ahhh, I remember back in the day... when it was morning.
Person 2: Ha. ICWYDT.
Person 1: Dude, can you go into your parents room?
Person 2: Uhhh... yeah, why?
Person 1: Because I need you to get my pants back from your mom.
Person 2: ... ICWYDT