Meaning:
`Be still, my beating heart' is an expression of excitement when seeing the
object of one's romantic affections.
Originally, it was used with the swooning earnestness of women's poetry of
the Romantic period. Now, it is more often used ironically, about suitors who
are indisputably unsuitable.
Background:
`Beating heart' has long been used to denote breathless excitement. John
Dryden used it with that meaning as early as 1697, in The works of Virgil:
"When from the Goal they start, The Youthful Charioteers with beating
Heart, Rush to the Race."
`My beating heart' was a stock expression for 18th century novelists and
poets. It is first recorded in Nicholas Rowe's Tamerlane, a tragedy, 1702:
"My beating Heart Bounds with exulting motion."
The earliest citation of the full `be still, my beating heart' comes from
William Mountfort's Zelmane, 1705:
"Ha! hold my Brain; be still my beating Heart."
...
- www.phrases.org.uk [edited]
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I first heard of the line in college from Sting the singer: "Stop before you
start. Be still, my beating heart" which I thought was cool. I was quick on the
uptake: the song was cautioning me to steer the mind away from girls, focus on
study, and earn good grades by which I had always been judged. The advice fell
flat shortly, however. I forgot the rest of the song but that verse stuck. Its
words are simple and I have never suspected part of it has been a stock
expression 300+ years old.