The word 'spyglass' is a transparent English compound from the early 18th century, directly combining the verb 'spy' and the noun 'glass'. 'Spy', from Old French 'espier' (to watch), ultimately of Germanic origin, conveys the act of secret or distant observation. 'Glass', from Old English 'glæs', refers to the material of the lens. The compound's logic is perfectly literal: it is a glass (lens) used for spying, or observing distant objects. It emerged as a common term for a portable, monocular telescope used primarily on ships before being largely superseded by the simpler word 'telescope'.

Structure: spy + glass - spy (Old French, Germanic): To watch, to observe. - glass (Old English): A transparent substance, here referring to a lens or optical device.