This phrase, or
the shorter version-"you wouldn't like me when I'm angry" was a
stock phrase on
The Incredible Hulk television series, starring
Bill Bixby. David (Bruce in the comic book series) Banner, mildmannered
nuclear scientist and all around
whipping boy would use these words when bothered by peers or the sneaky reporter trying to solve the mystery of David Banner's disappearance and the Hulk (big green monster thing-guy).
The phrase was used so often in the series it was finally added to the opening sequence that ran before each week's show.
It is now used as an ironic statement by people who act as if they have a bad temper, but can't seem to find it.