"A romantic bouncer," as The Alchemist
puts it. A person, most commonly female
and some years older than the people she is accompanying, who accompanies an unmarried heterosexual couple
so that they will not have a chance to be alone
together and do anything considered inappropriate
for an unmarried couple to be doing.
A nearly obsolete concept now in most Western cultures, but extremely important in the past to protect a woman's reputation. For example, in the book Gone With The Wind, Scarlett O'Hara is gossiping with a friend about Rhett Butler and is told: " Well, this Mr. Butler took a Charleston girl out buggy riding. I never did know who she
was, but I've got my suspicions. She couldn't have been very nice or she wouldn't have gone out with him in the late afternoon without a chaperon. And, my dear, they stayed out nearly all night and walked home finally, saying the horse had run away and smashed the buggy and they had gotten lost in the woods. And guess what—"
"I can't guess. Tell me," said Scarlett enthusiastically, hoping for the worst.
"He refused to marry her the next day!"