means A state of unease or agitation in which you can't settle, relax, or stay still — physically or mentally.

from Built from "rest" plus the negating suffix "-less" plus "-ness," so it's literally the quality of being without rest. "Rest" comes from Old English "ræst," meaning repose or a resting place — a word with deep Germanic roots, a cousin of German "Rast." "-less" is the same old negator that strips "hope" to "hopeless," and "-ness" bottles the whole condition into a noun. The word is exactly what it looks like: rest, denied, made into a name.