"spending or bestowing profusely," mid-15c., from Middle French lavasse (n.) "torrent of rain, deluge" (15c.), from Old French lavache, from laver "to wash," from Latin lavare "to wash" (from PIE root *leue- "to wash"). Related: Lavishly; lavishness.
"spend or bestow profusely," 1540s, from lavish (adj.). Related: Lavished; lavisher; lavishing.