The word was the creation of the cartoonist Mort Walker, by which he meant a comic-strip balloon containing symbols to indicate profanity. He first used it in 1964 in an article he wrote for the National Cartoonists Society in the US. He included it in his 1980 book The Lexicon of Comicana, a satire on cartoonists’ comic devices, which ironically became a textbook for art students.
Symbols under the heading Maledicta from Mort Walker’s book The Lexicon of Comicana
Under the heading of maledicta, the term that encompasses obscenity and other offensive language, he included four terms for graphically euphemistic devices: jarns, quimps, nittles and grawlixes. It would be hard to define a grawlix in a way that adequately distinguishes it from the others — they are all symbols and squiggles of various sorts — though his illustration for it is of what he calls “obstensibly obliterated epithets”, or scribbled-out words. Of the four, grawlix has gained the greatest currency, though it remains rare, and has largely taken over the territory of the other three.
Mort Walker’s book contains a lot of other terms that he invented for various comic-artist graphical conventions, including waftarom, squean, neoflect, spurl, plewd, vite, dite, hite, direct-a-tron (as well as throwatron, staggeratron, swishatron, sailatron ...), jigg, crottle-eyed, briffit, whiteope, indotherm and solrad. No, I’m not going to define any of them, not least because you can really only do it by illustrating them, as Walker did. Buy his book if you’re interested — it’s still in print.