type of hinged sash-window that swings open like doors, early 15c., originally "hollow molding, frame for glass," probably a shortening of Old French dialectal enchassement "window frame" (Modern French enchâssement), from en- "in," prefix forming verbs, + casse "case, frame" (see case (n.2)) + -ment. Or possibly from Anglo-Latin cassementum, from casse. The "window" sense is from 1550s in English. Old folk etymology tended to make it gazement.
The Irish surname is originally Mc Casmonde (attested from 1429), from a misdivision of Mac Asmundr, from Irish mac "son of" + Old Norse Asmundr "god protector."