Meaning

noun extensive tract of level open land

noun a basic knitting stitch

verb express complaints, discontent, displeasure, or unhappiness

originFrom Middle English pleyn, borrowed from Old French plein, from Latin plēnus (“full, filled, complete”). Ultimately from Proto-Italic *plēnos, from Proto-Indo-European *pl̥h₁nós (“full”). Doublet of plene, plenary, and full.

Etymology from Wiktionary, CC BY-SA 3.0