The phrase to steal someone’s thunder means: to use the ideas, policies, etc., devised by another person, political party, etc., for one’s own advantage or to anticipate their use by the originator.

It is said to have originated in an exclamation by the English critic and ineffective playwright John Dennis (1658-1734). After the early demise of one of his plays, for which a device for producing stage thunder had been invented, he supposedly heard this special effect being used during a subsequent performance of someone else’s play.

However, what play Dennis had written, what play he then saw, and exactly what he said vary according to the sources.

The English poet Alexander Pope (1688-1744) briefly mentioned this anecdote in his mock-heroic poem, The Dunciad: With notes variorum, and the prolegomena of Scriblerus. Written in the year, 1727 (1735 edition); about the line “With Thunder rumbling from the mustard-bowl”, Pope wrote:

The old way of making Thunder and Mustard ¹ were the same;  but since, it is more advantageously performed by troughs of wood with stops in them. Whether Mr. Dennis was the inventor of that improvement, I know not; but it is certain, that being once at a Tragedy of a new Author, he fell into a great passion at hearing some, and cry’d,
“S’death! ² that is my Thunder.”

¹ Metal balls in a mustard-bowl (i.e. a wooden bowl in which mustard seeds are pounded) were an established device for producing stage thunder.
² S’death: a euphemistic abbreviation of God’s death

The fourth volume of The lives of the poets of Great Britain and Ireland. By Mr. Cibber and other Hands (1753), by Robert Shiels and Theophilus Cibber, contains a detailed account:

[John Dennis’s] next dramatic production was Coriolanus, the Invader of his Country, or the Fatal Resentment, a Tragedy; altered from Shakespear [sic], and acted at the Theatre-Royal in Drury-Lane. This piece met with some opposition the first night; and on the fourth another play was given out. The second night’s audience was very small, though the play was exceedingly well acted. The third night had not the charges in money; the fourth was still worse, and then another play was given out, not one place being taken in the boxes for any ensuing night. The managers were therefore obliged to discontinue it.
[…]
Mr. Dennis happened once to go to the play, when a tragedy was acted, in which the machinery of thunder was introduced, a new artificial method of producing which he had formerly communicated to the managers. Incensed by this circumstance, he cried out in a transport of resentment,
‘That is my thunder by G—d; the villains will play my thunder, but not my plays.’

According to William Shepard Walsh (1854-1919) in A Handy-Book of Literary Curiosities (Philadelphia, 1893), John Dennis’s play was Appius and Virginia, and it was at a representation of Macbeth that he

heard his own thunder made use of. “Damn them!” he cried, rising in a violent passion, “they will not let my play run, but they steal my thunder!”

Walsh writes that he quotes the English literary scholar and anecdotist Joseph Spence (1699-1768), but the latter’s published works do not seem to contain those details.

In any case, Walsh’s version of the story has now become the accepted truth: many ‘etymologists’ do not even bother to mention his name, and the fact that they copy from one another creates a semblance of veracity.

The phrase to steal someone’s thunder is attested in the relevant sense in the first half of the 19^th century only. The earliest instance that I have found is from Miseries of a leading article manufacturer, published in the Inverness Courier (Scotland) of 21^st February 1827:

Party is diverting away from its legitimate channels, and Whig and Tory are almost defunct. Whether Mr Canning ³ has stolen Mr Brougham’s ⁴ thunder, or not, is nothing to me: the melancholy fact is undeniable, that the Ministry and the Opposition have come to a most lamentable approximation on great leading questions.

³ George Canning (1770-1827), British statesman and Tory politician
⁴ Henry Brougham (1778-1868), British statesman and Whig politician

If the phrase did originate in John Dennis’s exclamation, it is difficult to explain the gap between the period of the anecdote and that of the first recorded uses of to steal someone’s thunder. A common explanation is that the phrase was used in conversation, particularly within theatrical circles, long before its appearance in print. An article published in the Dundee, Perth and Cupar Advertiser (Scotland) of January 1847 seems to confirm this; it mentions

the story of John Dennis, who, upon witnessing the players appropriating his invention for raising thunder in the theatre, exclaimed, “they have stolen my thunder.”

However, it is also possible that the phrase did not originally refer to the anecdote but to stealing thunder from the Roman sky god Jupiter. At least, nothing in its earliest attestations excludes this possibility.

In The American Gentleman’s Guide to Politeness and Fashion (New York, 1858), the American author Margaret Cockburn Conkling (1814-90), writing under the pen name of Henry Lunettes, associated to steal someone’s thunder, albeit in a slightly different sense, with Jupiter:

It is said that Sheridan ⁵, one of the most celebrated wits and conversationists of his day, prepared himself for convivial occasions, like an intellectual gladiator, ready to enter the lists in a valiant struggle for supremacy. […]
Sheridan’s practice was, to make brief notes, before going into society, of appropriate topics and witticisms for each occasion, upon which he relied for sustaining his reputation as a boon companion and accomplished talker. There is a good story told of his being exceedingly nonplussed, on some important occasion, by having his memoranda purloined by a friend, who, while waiting to accompany the wit to an entertainment to which both were invited, stole his thunder from his dressing-table, where it had been placed in readiness. The unlucky literary Boanerges ⁶ was as powerless as Jupiter robbed of his bolts!

⁵ Richard Brinsley Sheridan (1751-1816), Irish playwright and politician
⁶ Boanerges: a nickname applied by Jesus to James and John in the gospel of Mark, 3:17, hence: a loud vociferous preacher or orator

 

Other words or phrases of theatrical origin:
to be decent (sufficiently clothed to see visitors)
malapropism
claptrap
cloak-and-dagger
Mrs Grundy
Paul Pry
Mummerset
simon-pure
fedora
Box and Cox
Hamlet without the Prince
slapstick
old chestnut
to play to the gallery
bums on seats