A portmanteau word of something that is simultaneously funny or hysterical (hyst-) and awful or terrible (-terrible).
Man, did you see that car full of clowns get hit by that pie-truck? Hysterrible.
Having both hysterical and terrible qualities, each of which intensifies the other. Describes something you laugh at, even though you know you shouldn't.
Have you heard the song "Special Olympics" by Stephen Lynch? Its hysterrible.