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To slump or fall over, typically from fainting or death. The phrase is typically used in a humorous way, especially when it refers to someone dying. It was so hot that I thought I would keel over and pass out if I didn't get some air soon. Grandpa always used to tell us this silly story about the time when his friend ate a sour pickle and keeled over and died right then and there.
keel something over
Rur. to push something over. He leaned on the flimsy wall and keeled it right over. The high wind keeled over that sorry old fence.
[for a person] to fall over or fall down in a faint or in death. It was so hot in the room that two people just keeled over.
Collapse, as if in a faint; also, faint. For example, When she heard the awful news, she keeled over. This term alludes to a vessel rolling on its keel and capsizing. [Mid-1800s]
1. To fall over; capsize: The ship keeled over when it hit the iceberg.
2. To collapse or fall into or as if into a faint: I keeled over when I heard the bad news.