Bought the farm
To die, particularly in an accident or military action.
The origin of this phrase is uncertain. It is 20th century and all the early references to it relate to the US military. The New York Times Magazine, March 1954, had a related phrase, in a glossary of jet pilots' slang:
"Bought a plot, had a fatal crash."
That clearly refers to a burial plot. The 'bought' in that case probably doesn't suggest any actual or potential purchase, but to an earlier use of 'bought', that is, being killed. This dates back to at least the early 20th century. This example from 1943 isn't the earliest, but it does make the meaning explicit. It's from Cyril Ward-Jackson's It's a piece of cake; or, R.A.F. slang made easy:
"He's bought it, he is dead - that is, he has paid with his life."
Specific references to 'the farm' come a little later. There are reports of the phrase being in use in the US military from 1955 onward. Here's a citation from 1963, in Ed Miller's Exile to the Stars:
"The police dispatcher says a plane just bought the farm."
There are a few suggested derivations for the phrase. One, put forward in a 1955 edition of American Speech, is the idea that when a jet crashes on a farm the farmer may sue the government for compensation. That would generate a large enough amount of money to pay off the farm's mortgage. Hence, the pilot paid for the farm with his life.
The second theory is that military men might dream of returning from the battlefront and settling down with a family to a peaceful life down on the farm. If someone were killed his colleagues might say, 'well, he bought the farm early', or similar. Well, yes they might, and there are numerous sentimental US films where dialogue like that wouldn't be out of place. That's not to say the phrase was coined that way though.
A third suggestion is the idea that, if a serviceman was killed in action, his family would receive a payout from the insurance that service personnel were issued with. This would be sufficient to pay off the family mortgage.
My twopenneth is on the last explanation but, given that we don't have the full evidence, that's just speculation.