Meaning

noun a stupid person; these words are used to express a low opinion of someone's intelligence

noun very large carnivorous sea turtle; wide-ranging in warm open seas

originFrom logger (“(dialectal) heavy wooden block secured to a horse's leg to prevent it from straying”) + head. Logger was apparently coined because its sound connotes a clumsy, heavy object; compare log (“trunk of a dead tree; bulky piece of wood”).

Etymology from Wiktionary, CC BY-SA 3.0