Be still, my beating heart
Expression of excitement when seeing the object of one's romantic affections.
Originally used with the swooning earnestness of women's poetry of the Romantic period. Now more often used ironically, about suitors who are indisputably unsuitable.
'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."
The expression, and the comic manner in which it is now delivered, were brought to a wide public in Gilbert and Sullivan's opera HMS Pinafore, 1878:
Aye, even though Jove's armoury were launched at the head of the audacious mortal whose lips, unhallowed by relationship, dared to breathe that precious word, yet would I breathe it once, and then perchance be silent evermore. Josephine, in one brief breath I will concentrate the hopes, the doubts, the anxious fears of six weary months. Josephine, I am a British sailor, and I love you!
Sir, this audacity!
(Aside.) Oh, my heart, my beating heart!
(Aloud.) This unwarrantable presumption on the part of a common sailor!