In this week’s Dispatches from The Secret Library, Dr Oliver Tearle explores the meaning and origins of a famous Shakespeare phrase
‘Hoist with one’s own petard’. The expression is well-known, and its meaning is fairly clear to most people: it describes someone who has been scuppered by their own schemes, someone who has come a-cropper because of some mischief they intended against others. But what is a ‘petard’, and where does it come from?
As with so many phrases in common use, such as ‘the be-all and end-all’, ‘salad days’, ‘foregone conclusion’, ‘the world’s my oyster’, ‘the course of true love’, ‘dead as a doornail’, ‘good riddance’, ‘milk of human kindness’, ‘one fell swoop’, and many others, the phrase appears to have originated in the work of Shakespeare. And like many other phrases in common use, such as ‘more honoured in the breach than the observance’, ‘to the manner born’, ‘something is rotten’, ‘cruel to be kind’, ‘brevity is the soul of wit’, ‘in my mind’s eye’, and ‘frailty, thy name is woman’, the phrase ‘hoist with one’s own petard’ is derived specifically from one Shakespeare play: Hamlet.
Hamlet uses the phrase in Act 3 Scene 4 of the play, while Hamlet is talking with his mother, Gertrude, and insinuates that he is going to outwit his two ‘schoolfellows’, Rosencrantz and Guildenstern, who have been ordered to take him to England and have him killed:
There’s letters sealed; and my two schoolfellows,
Whom I will trust as I will adders fanged,
They bear the mandate; they must sweep my way
And marshal me to knavery. Let it work,
For ’tis the sport to have the enginer
Hoist with his own petard; and ’t shall go hard
But I will delve one yard below their mines
And blow them at the moon. O, ’tis most sweet
When in one line two crafts directly meet.
Hamlet’s assertion that ‘’tis the sport to have the enginer / Hoist with his own petard’ means that it will be good to see the schemer (i.e. 
Claudius, Hamlet’s villainous uncle) defeated by his own scheme. Hamlet’s metaphor is military in flavour: an ‘enginer’ (similar to ‘engineer’) is a maker of engines, including bombs, so the sentiment is that a maker of bombs would end up being blown up by his own bomb.
This is what a ‘petard’ is: a bomb. The word ‘hoist’ sometimes causes people problems, and they come away with the erroneous impression that ‘hoist with one’s own petard’ suggests getting tied up in one’s own rope (because of the more common meaning of the word ‘hoist’, e.g. hoisting a flag up a flagpole).
Indeed, the Oxford English Dictionary, in its entry for ‘hoist’, does not record the sense of ‘blow up’ or ‘be blown up’: merely ‘raised aloft’ or ‘placed on high’. (Shakespeare uses the verb ‘hoist’ with this meaning in Antony and Cleopatra: ‘Let him take thee, / And hoist thee up to the shouting Plebeians.’) Meanwhile, however, the entry for ‘hoise’, a similar if less familiar verb which also means to raise something aloft with a rope or a pulley, contains its own separate sub-entry for ‘hoist with one’s own petard’, quoting Shakespeare’s line as the earliest instance of this use. The mental image we are supposed to see, then (in our mind’s eye), is of someone being blown up high into the air by their own bomb or ‘petard’.
But what about that word ‘petard’? The OED offers several different senses of this word in its entry for ‘petard’, but the earliest and chief meaning is: ‘A small bomb made of a metal or wooden box filled with powder, used to blow in a door, gate, etc., or to make a hole in a wall.’ That makes perfect sense, and seems to explain everything.
Or does it? In the second Quarto printing of Hamlet, the word is spelt not ‘petard’ but ‘petar’. This may be merely evidence of the vagaries of English spelling in the late Elizabethan and early Jacobean period. But it may also have been a bit of cheeky wordplay. The other main sense of ‘petard’ which the OED defines is: ‘A kind of firework that explodes with a sharp report; a cracker.’ A small bomb, if you will, that emits a sound and a slight odour. The word ‘petard’ is derived from the Middle French pet, which means, and there’s no polite way of saying this, a fart. As Tom Burnam points out in More Misinformation
So, for all that, although ‘hoist with his own petard’ is used by Hamlet to suggest the idea of being blown up by one’s own bomb, Shakespeare’s original audience might also have appreciated a secondary meaning, about having to endure the unpleasant odours caused by one’s own expulsion of intestinal gases. Or, to put it more simply: did Shakespeare invent the sentiment ‘whoever smelt it, dealt it’?
Oliver Tearle is the author of The Secret Library: A Book-Lovers’ Journey Through Curiosities of History