Never argue with a fool; onlookers may not be able to tell the difference.

Mark Twain (1835-1910) American writer [pseud. of Samuel Clemens]
(Spurious)

Frequently attributed to Twain and also to Immanuel Kant (but never, in either case, with any citation). The phrase first makes recognizable (if anonymous) appearance in the late 19th Century; attributions to Twain begin in the late 1990s. See also Proverbs 26:4. For more discussion (and a shout-out to WIST) see here.