"mode of living," especially a working agreement between contending parties, 1844, Latin, literally "way of living or getting along" (see modus).
Modus vivendi is any temporary compromise that enables parties to carry on pending settlement of a dispute that would otherwise paralyse their activities. [Fowler]
in couch-grass, 1570s; a corruption of Old English cwice "living, alive" (see quick (adj.)).