also spy-glass, "telescope, field-glass," 1706, from spy (v.) + glass (n.). Spying-glass is from 1680s.
Entries linking to spyglass
spy v.
mid-13c., "to watch stealthily," from Old French espiier "observe, watch closely, spy on, find out," probably from Frankish *spehon or some other Germanic source, from Proto-Germanic *spehon- (source also of Old High German *spehon "to look out for, scout, spy," German spähen "to spy," Middle Dutch spien), the Germanic survivals of the productive PIE root *spek- "to observe." Old English had spyrian "make a track, go, pursue; ask about, investigate," also a noun spyrigend "investigator, inquirer." Italian spiare, Spanish espiar also are Germanic loan-words. Meaning "to catch sight of" is from c. 1300. Children's game I spy so called by 1946.
glass n.
Old English glæs "glass; a glass vessel," from Proto-Germanic *glasam "glass" (source also of Old Saxon glas, Middle Dutch and Dutch glas, German Glas, Old Norse gler "glass, looking glass," Danish glar), from PIE root *ghel- (2) "to shine," with derivatives denoting bright colors or materials. The PIE root also is the ancestor of widespread words for gray, blue, green, and yellow, such as Old English glær "amber," Latin glaesum "amber" (which might be from Germanic), Old Irish glass "green, blue, gray," Welsh glas "blue."
Restricted sense of "drinking glass" is from early 13c. and now excludes other glass vessels. Meaning "a glass mirror" is from 14c. Meaning "glass filled with running sand to measure time" is from 1550s; meaning "observing instrument" is from 1610s.