Grawlix are the garbage typographic characters used in comic books to denote characters swearing, by $#&%@! The term was coined by Mort Walker, of Beetle Bailey and Hi and Lois fame, in his book the Lexicon of Comicana; it has no derivation, but appears to be essentially onomatopoetic. In the book, Walker makes up many of these words, half in jest; however, to my knowledge only »grawlix« has caught on in actual usage. In fact, the adopted usage is inaccurate to that intended by Walker: in the book, he calls each character a grawlix, with a mass of them being pluralized grawlixes; but in common use (and as I have it here), grawlix is an uncountable noun, like rice or water.
Uderzo, famed artist of Asterix, draws notably elaborate grawlix, which often contain little jokes of their own (a Roman soldier's swearing contains a skull and crossbones with a legionary's helmet; the Ostrogoth swearing back at him produces a skull with a pickelhaube).