One of the mysteries of the English language finally explained.
2A hole or opening for looking through; a small window or window-like opening. Formerly also: an opening in a wall through which missiles may be discharged. Compare earlier oillet. Now rare.
Late 16th century. From eyelet + hole, perhaps as a remodelling of earlier oillet-hole.
eyelet hole/ˈʌɪlɪt ˌhəʊl/
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