Answer: Obscenities

As early as 1902, cartoonists were using mixed typographical symbols to represent the letters of obscene words in the dialogue of their characters. Adult readers could easily translate “That guy, what a *#($ head!” into more colorful language, but the random characters instead of the actual word hypothetically kept obscenities away from children and, more practically, kept the censors away from the cartoonists.

The term “grawlix” to refer to this style of mixed typography was coined in 1964 by cartoonist Mort Walker in his extensive treatise on the medium entitled Lexicon of Comicana.