What does ISJ mean?
inside joke
Other definitions of ISJ:
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How to use the term ISJ:
There are no example uses of ISJ at this time. We would greatly appreciate your contribution if you would like to submit your own!
More slang terms:

YKK
Yoshida Kogyo Kabushikikaisha (zipper manufacturer)

DONCHA
Don't you

BOED
Book of Exalted Deeds

QI
Quite Interesting