Stands for "I should've been invited". An acronym used to describe excessive erratic behavioral shifts when the offending party is not privy to being invited somewhere due to her ever-sad countenance, overall gloom and perpetual air of malaise.
Scott: "Paul's at the mall, I just had to come back and get something."
Scalene <super off the wall excessively offended with a red face and teary eyes>: "At the mall? I should've been invited."
Scott: "I don't have time for this now." <muttering in orcish; "fcuking ISBI">