loggerheadyoudaoicibaDictYouDict[loggerhead 词源字典]
loggerhead: [16] Loggerhead originally meant much the same as blockhead – a stupid person with a block of wood for a head (in Shakespeare’s Love’s Labour’s Lost (1588), Berowne calls Costard a ‘whoreson loggerhead’). The first part of it probably represents logger ‘block for hobbling a horse’, which in turn was based on log, but how it came to be used in the phrase at loggerheads, meaning ‘in conflict’ and first recorded in 1831, is unclear.

Perhaps the underlying image is of two stupid people having an in-your-face argument, but loggerhead was also used over the centuries for various bulbous-ended objects, including a long-handled tool for melting pitch, and it could be that a fight is being invoked in which these fearsome articles are being used as weapons.
[loggerhead etymology, loggerhead origin, 英语词源]