Knock into a cocked hat
To beat severely.
Of course, 'knocked into a cocked hat' only makes any sense if you know what cocked hats are. These were hats, popular at the end of the 18th century, that had turned up (i.e. cocked) brims. They were usually tricorn (i.e. three-cornered) hats and were often worn as part of some form of official regalia. They were first mentioned in print in William Wycherley's play The Gentleman Dancing-master, 1673:
Instead of laced coats, belts, and pantaloons,
Your velvet jumps, gold chains, and grave fur gowns
Instead of periwigs, and broad cocked hats,
Your satin caps, small cuffs, and vast cravats.
The 'Toby jugs' that are still commonplace household ornaments in the UK were usually modelled wearing cocked hats. Toby was Toby Phillpot, not a real person but a stylized version of the brown earthenware 'fill pot' jugs that were used in English pubs to refill tankards before beer pumps were invented. The figures are usually shown holding a fill pot.
Some etymologists have speculated that 'knocked into a cocked hat' derived from the game of Ninepins. The theory was that when the pins were struck so that just three were left, in a triangular cocked hat shape, they were too badly out of shape for the game to be won on one throw. Alternatively, the phrase is said to derive from the American version of the game, which used just three skittles and was known as 'Cocked hat'. Theory appears to be the right word for these explanations, as there is no supporting evidence to back them up. It appears that the American version of skittles was named after the hat but was not otherwise connected with the phrase. A more likely derivation is that 'to knock someone into a cocked hat' was simply to pummel them so badly as to alter their normal appearance.
Cocked hats were also worn in America in the 19th century and 'knocked into a cocked hat', despite the hat's link with Olde Englande's town criers, beadles and taverns, is actually an American phrase. It is first found in the 1830s, as in this example from James Kirke Paulding's novel The banks of the Ohio; or, Westward ho!, 1833:
I told Tom - I'd knock him into a cocked-hat, if he said another word.
Another example, from just a few years later, in the New York State newspaper, The Rural Repository, 1837 is also worth including as it conveys the apparent ferocity intended with the use of the phrase:
'Blood and vengeance!' exclaimed Boniface, 'get out of my house, you varmints, or I will knock you into a cocked hat, and gormandize you!'
This was probably more of a concern to Boniface's erstwhile opponents than it is to us, but 'gormandize' means 'eat voraciously'.
See other phrases that were coined in the USA.