The word 'ear' for the organ of hearing is one of the most broadly attested body-part terms in the Indo-European family, appearing with consistent meaning in virtually every branch. It descends from Old English 'ēare,' from Proto-Germanic *ausō, from PIE *h₂ous- (ear). The Latin cognate 'auris' gave English the learned adjective 'aural' (pertaining to the ear) and the medical term 'auscultation' (listening to internal body sounds with a stethoscope, from Latin 'auscultāre,' to listen).
The phonological development from PIE *h₂ous- to English 'ear' is instructive. The PIE laryngeal *h₂ was lost in Proto-Germanic, leaving *aus-. The *au diphthong developed into Old English 'ēa' (a process shared by many words), producing 'ēare.' The Middle English form 'ere' was later respelled 'ear' — a spelling that obscures the original diphthong but has been standard since the fifteenth century.
It is important to distinguish this 'ear' from the homophone 'ear' meaning 'a spike of grain' (as in 'an ear of corn'). These are entirely unrelated words. The grain-ear comes from Old English 'ēar' (spike, ear of grain), from Proto-Germanic *ahaz, from PIE *h₂eḱ- (sharp, pointed) — the same root that gave Latin 'acus' (needle) and 'acuere' (to sharpen). The coincidence of their modern forms is a historical accident.
The compound 'earwig' is one of the oldest and most atmospheric words in English entomology. It comes from Old English 'ēarwicga,' literally 'ear-creature' or 'ear-beetle' (from 'wicga,' an insect, possibly related to 'wiggle'). The name reflects an ancient and widespread folk belief that earwigs crawl into the ears of sleeping people and bore into the brain. The belief is found across European cultures — German 'Ohrwurm' (ear-worm), French 'perce-oreille' (ear-piercer) — but it is entirely without medical basis. The name has survived
'Earmark' originally referred to a literal mark cut into the ear of livestock to identify ownership — a practice dating back millennia. The figurative sense of 'earmark' (to designate for a specific purpose) dates from the seventeenth century. 'Earring' is from Old English 'ēarhring' (ear-ring). 'Eavesdrop' contains no ear-element: it comes from Old English 'yfesdrype' (the dripping from the eaves), and an eavesdropper was someone who stood under the eaves close enough to the wall to overhear conversations inside.
The Indo-European cognates include Greek 'oûs' (οὖς, ear), Sanskrit 'karṇa' (from a different root), Lithuanian 'ausis,' Old Church Slavonic 'ucho,' and Armenian 'unkn.' The breadth of this cognate set across the family demonstrates that *h₂ous- was the primary, unmarked word for 'ear' in the proto-language.