loggerhead

Concise Oxford English Dictionary © 2008 Oxford University Press:
loggerhead/ˈlɒɡəhɛd/

▶noun

• 1 (also loggerhead turtle) a large-headed reddish-brown turtle of warm seas. [Caretta caretta.]

• 2 archaic a foolish person.

– phrases
at loggerheads in violent dispute or disagreement. [perh. a use of loggerhead in the 17th-cent. sense ‘long-handled iron instrument for heating liquids’ (when wielded as a weapon).]

– origin C16: from dial. logger ‘block of wood for hobbling a horse’ + head.