Top definition
An acronym for In Your Butt, Yup.
IYBY is a variant of the "that's what she said" response. It has multiple applications. You can also alter and utilize portions of the phrase: "inside you", or, when shortened "in you".

Person 1: I loooove 5 guys.
Person 2: IYBY

Another example of partial utilization:

Person 1: Yeah, I'm almost ready to take this roadtrip, I just have to finish packing up.
Person 2: Your butt, yup.
via giphy
by Bigboy608 July 19, 2018
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