Q: As someone who ranks high on the perspiration index, I was wondering when the phrase “don’t sweat it” came about.
A: “Don’t sweat it” first showed up in print about 50 years ago, according to the Oxford English Dictionary, but we’ve found a similar expression that appeared in writing 50 years before that.
The OED describes “don’t sweat it” as American slang for “don’t worry.” The dictionary says a positive colloquial version, “to sweat,” means “to experience discomfort through anxiety or unease.”
The earliest example for “don’t sweat it” in Oxford is from a 1963 issue of the journal American Speech: “Don’t sweat it means ‘don’t worry about it.’ ”
However, we’ve found this similar usage in the Dec. 12, 1914, issue of Happy Days, a New York weekly newspaper:
“ ‘What’s the meeting for, anyway?’ said Paul Braddon. ‘Keep your shirt on, and don’t sweat it off,’ said Deacon Small.”
The first positive citation (grammatically speaking) in the OED is from The Hungarian Game, a 1973 espionage thriller by Roy Hayes:
“ ‘Hold off for a moment. I want to watch him sweat.’ ‘The guy’s about to faint from pain.’ ”
As you can imagine, the verb “sweat” in its literal sense is very old, dating back to Anglo-Saxon times. The infinitive was swætan in Old English and meant (as it does today) to emit perspiration through the pores of the skin.
The first example in the OED is from the Venerable Bede’s Ecclesiastical History of the English People (circa 900):
He swa swiðe swætte swa in swole middes sumeres (“He so sweated strongly in the mid-summer heat”).
The Chambers Dictionary of Etymology says that “sweat” is ultimately derived from the proto-Germanic root swaita-, and that it has given us such words and phrases as “sweater” (1882, the garment), “sweatshop” (1889), and “sweatshirt” (1929).
from Grammarphobia http://www.grammarphobia.com/blog/2016/04/sweat.html