The meaningless typographical gibberish used by cartoonists as a stand-in for unprintable dialogue (usually profanity, but sometimes just meant to convey unintelligible incoherence).
The term (originally singular; plural grawlixes) was coined by American cartoonist Mort Walker, creator of Beetle Bailey, in a 1964 satire for the National Cartoonists Society entitled "Let's Get Down to Grawlixes". Walker later expanded on the theme in a 1980 book, The Lexicon of Comicana, in which he coined "technical" terms for other common visual shorthands, such as plewds (flying sweatdrops) and emanata (^^^).
