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