The OED says the term is two words (“worry wart”) and is colloquial, chiefly in North America. It says the first appearance was in 1956.

Is worrywart an idiom?

informal Someone who is excessively, compulsively, or needlessly worrisome or fretful. Sometimes spelled as two words. That's the last time I go backpacking with John. He was just an insufferable worrywart for the whole trip!

What do you call a worrywart?

In this page you can discover 11 synonyms, antonyms, idiomatic expressions, and related words for worrywart, like: worrier, neurotic, nervous person, cassandra, nervous type, old maid, doomsayer, pessimist, hope, fuss-budget and fusspot.