Old English lauþr "foam, washing soda," from Proto-Germanic *lauthran (cognates: Old Norse lauðr "washing soap, foam"), from PIE *loutro- (cognates: Gaulish lautron, Old Irish loathar "bathing tub," Greek louein "to bathe," Latin lavere "to wash"), which is from root *leu(e)- "to wash" + instrumentative suffix *-tro-. The modern noun might be a 16c. redevelopment from the verb. Meaning "violent perspiration" (especially of horses) is from 1650s. Meaning "state of agitation" (such as induces sweating) is from 1839.

Old English laþran, from Proto-Germanic *lauthrjan (source also of Old Norse leyðra "to clean, wash;" see lather, n.). Related: Lathered; lathering.