What's the meaning of the phrase 'Het up'?
To be het-up is to be extremely agitated.
What's the origin of the phrase 'Het up'?
Het up is an odd expression as het isn't a word we use in any other context. Actually het is a shortened form of heated and has been used that way since the 14th century, although it fell out of use as a standalone word in the late 19th century.
That simple literal meaning of het up as hot is first recorded in the US newspaper The Freeborn County Standard, July 1884:
"Set it right down here by the fire Susan, so 't'll get het up before you knead it into loaves."
The first record I can find of someone using het up to mean agitated is in a work by the American physician S. W. Mitchell in 1886:
"I don't het up easy."