O.E. lauþr "foam, washing soda," from P.Gmc. *lauþran (cf. O.N. lauðr "washing soap, foam"), from PIE *loutrom (cf. Gaul. lautron, O.Ir. loathar "bathing tub," Gk. louein "to bathe," L. lavere "to wash"), which is from base *lou- "to wash" + instrumentative suffix *-tro-. Meaning "state of agitation" (such as induces sweating) is from 1839. The verb is from O.E. laþran, from P.Gmc. *lauþrjan.