Here's what we know:
* Early Usage: The earliest recorded use of the phrase appears in 1768, in the play *The Rivals* by Richard Brinsley Sheridan. In the play, a character named Bob Acres says, "I'll whisper sweet nothings in her ear, and talk of love, and stars, and all that sort of thing."
* Romantic Context: The phrase was almost immediately associated with romantic courtship. Whispering was a common way for lovers to communicate privately, and "sweet nothings" referred to endearing but often nonsensical words spoken to endear oneself to another.
* Evolution of Meaning: Over time, the phrase became a more general term for romantic compliments and expressions of affection, even if they weren't actually whispered.
While the precise origin of the phrase is a mystery, it's clear that "whisper sweet nothings" has been a part of the English language for centuries and has become a well-known idiom for romantic gestures.