Stories We Like: A Guide to the Comma
1580s, "stupid person, blockhead," perhaps from dialectal logger "heavy block of wood" + head (n.). Later it meant "a thick-headed iron tool" (1680s), a type of cannon shot, a type of turtle (1650s). Loggerheads "fighting, fisticuffs" is from 1670s, but the exact notion is uncertain, perhaps it suggests the heavy tools used as weapons. The phrase at loggerheads "in disagreement" is first recorded 1670s.