In lumbering, a log which is so caught or wedged that a jam is formed and held by it.(noun)
The Century Dictionary (Public Domain)
Use "key-log" in a sentence
"One of the foremen of Benbow Camp -- the best ax wielder of the crew -- ran out on the boom to a point near the middle of the frothing stream and began cutting the key-log."
"Moncrossen, Stromberg, Fallon, and one other to search for the key-log."
"I landed on the island and watched with great interest the men as they pried, twisted and tumbled the pile to get at the key-log which, found and loosened, would send the heap into the water."