ITYSW stands for I Thought You Said Weast. This became popular on SpongeBob SquarePants, a popular cartoon on Nickelodeon.
ITYSW - I Thought You Said Weast
Him: You were supposed to go East!
You: East!? I Thought You Said Weast!!!
Him: You were supposed to go East!
You: East!? I Thought You Said Weast!!!
by Sir LoanStark May 19, 2021