Used to describe something being an abysmal failure or disgrace to humanity. The ultimate letdown. A great word to use when "disappointed" just isn't enough.
Lorii: OMGGGG I failed my econ midterm QQ
Moose: LQL, you've really dessipointed me.
Lorii: OMGGGG I haven't watched the new pokemon episode yet
Moose: LQL, what a dessipointment.