brat

First recorded in 1500–20; perhaps transferred use of Middle English brat “cloak of coarse cloth, rag,” Old English bratt “cloak,” from Celtic; compare Irish brat “mantle, cloak”

Dr. Johnson never minced words: in his Dictionary (1755) he defines brat as “A child, so called in contempt.” A few years earlier, in 1750, in one of his articles for the Rambler, No. 15, Dr. Johnson writes, “The children are out at nurse in villages as cheap as any two little brats can be kept, nor have I ever seen them since; so he has no trouble about them.” Brat was and still is not always used in contempt, but at the time, the word usually implied insignificance or poverty, as in beggar’s brat.
Brat probably comes from a Celtic language: in Irish, bratt means “a cloak, a cloth (especially as a covering for one’s body)”; in Welsh, brethyn means “cloth.” In Old English, bratt “cloak” is used in the Lindisfarne Gospels dating to the early 8th century, composed on Lindisfarne, an island off the east coast of Northumberland. Chaucer uses brat “a cloak of cloth” in The Canterbury Tales (after 1394). In British dialects of the Midlands and North, brat means “a woman’s or child’s apron, pinafore.”
The American military slang term army brat, “the son or daughter of a career officer or enlisted person,” dates to the early 1930s. Brat pack, “a successful, highly confident, and often close-knit group of famous young people, especially actors,” dates to the mid-1980s.