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.