Lavish

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late Middle English (as a noun denoting profusion): from Old French lavasse ‘deluge of rain’, from laver ‘to wash’, from Latin lavare .

wiktionary

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From Middle English *lavish, laves, *lavaus, lavage(“extravagant, wasteful”), of uncertain origin. Perhaps from Old French lavasse(“torrent of rain”), or derived from Middle English laven(“to pour out”). See lave.

etymonline

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lavish (adj.)

"spending or bestowing profusely," mid-15c., laves, from Old French lavasse,lavache (n.) "a torrent of rain, deluge" (15c.), from laver "to wash," from Latin lavare "to wash" (from PIE root *leue- "to wash"). Related: Lavishly; lavishness.

lavish (v.)

"spend or bestow profusely," 1540s, from lavish (adj.). Related: Lavished; lavisher; lavishing.