From Middle English lather, from Old English lēaþor (“a kind of nitre used for soap, soda”), from Proto-West Germanic *lauþr, from Proto-Germanic *lauþrą (“that which is used for washing, soap”), from Proto-Indo-European *lówh₃trom (“that which is used for washing”), from *lewh₃-, *lowh₃- (“to wash, bathe”). Cognate with Swedish lödder (“lather, foam, froth, soap”), Icelandic löður (“foam, froth, a kind of niter used for soap”), Old Irish loathar (“wash-basin”), Ancient Greek λουτρόν (loutrón, “a bath, wash-room”), Latin lavō (“to wash”), Albanian laj (“to wash”), Ancient Greek λούω (loúō). More at lye. Compare typologically Russian мы́ло (mýlo) (a nomen instrumenti of мыть (mytʹ)).
1
The foam made by rapidly stirring soap and water.
Stately, plump Buck Mulligan came from the stairhead, bearing a bowl of lather on which a mirror and a razor lay crossed.1922 February, James Joyce, “[Episode 1: Telemachus]”, in Ulysses, Paris: Shakespeare and Company, […], →OCLC, part I [Telemachia], page 3:
Etymology From Middle English letheren, from Old English līeþran, from Proto-West Germanic *lauþrijan, from Proto-Germanic *lauþrijaną (“to soap, wash”). Cognate with Old Norse leyðra (“to wash, lather”).
1
To cover with lather.
I lathered my body with lavender-scented soap.
2
To beat or whip.
3
To form lather or froth, as a horse does when profusely sweating.
I woke Corporal Honda to see to the horse. Heavily lathered and breathing hard, it had obviously come a long way at high speed.1997, Haruki Murakami, translated by Jay Rubin, The Wind-Up Bird Chronicle, New York: Alfred A. Knopf, Inc.; republished New York: Vintage Books, 1998, →ISBN, page 147: