How Well Do You Know English Slang?
c.1600s, originally a noun (1560s), from Middle French crotesque (16c., Modern French grotesque), from Italian grottesco, literally "of a cave," from grotta (see grotto). The usual explanation is that the word first was used of paintings found on the walls of basements of Roman ruins (Italian pittura grottesca), which OED finds "intrinsically plausible." Originally "fanciful, fantastic," sense became pejorative after mid-18c. Related: Grotesquely; grotesqueness.