"habitually complaining; expressing complaint," c. 1400, querelous, from Old French querelos "quarrelsome, argumentative" and directly from Late Latin querulosus, from Latin querulus "full of complaints, complaining," from queri "to complain," from Proto-Italic *kwese-, of uncertain etymology, perhaps, via the notion of "to sigh," from a PIE root *kues- "to hiss" (source also of Sanskrit svasiti "to hiss, snort"), which is not very compelling, but no better etymology has been offered.
It retains the original vowel of quarrel (n.1). Related: Querulously; querulousness.
<a href="https://www.etymonline.com/word/querimonious">Etymology of querimonious by etymonline</a>
Harper, D. (n.d.). Etymology of querimonious. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/querimonious