late 14c., transeyn "crossbeam spanning an opening, lintel," probably by dissimilation from Latin transtrum "crossbeam" (especially one spanning an opening), from trans "across, beyond" (from PIE root *tere- (2) "cross over, pass through, overcome") + instrumental suffix -trum. Meaning "small window over a door or other window" is first recorded 1844.

framed part of a window, into which the panes are fitted, 1680s, sashes, a mangled Englishing of French châssis "frame" of a window or door (see chassis).

The word was mistaken as a plural and further mangled by loss of the -s by 1704. Sash-door, one having panes of glass to admit light, is by 1726; sash-weight, attached by cords to either side of a sash to balance it and make it easier to raise and lower, is attested by 1737.

"mechanism that clears condensed water vapor from the window of an automobile," by 1962, from agent noun from defog (v.) which is attested from 1945 (implied in defogging); see de- + fog.

1870 in the anatomical sense, noun of action from Latin fenestrare, from fenestra "window, opening for light," a word perhaps from Etruscan (see defenestration). Meaning "arrangement of windows" as a design element in architecture is from 1846. Related: Fenestrated.