Meaning

adjective being in a tense state

adjective impatient especially under restriction or delay

originModification of earlier restiff, from Middle English restyf, from Old French restif, from rester (“to stay, remain”), from Latin restō. * Shares an etymology with rest ("to remain," obsolete) *Merriam-Webster states that this word was originally used to describe horses that disobeyed commands. Presumably, then, the word came to mean fidgety or anxious more broadly.

Etymology from Wiktionary, CC BY-SA 3.0