Origin: 1325–75; late Middle English traunsum, traunsom,Middle English transyn, probably alteration (by association with trans-) of traversayn < Old French traversin crosspiece, derivative of travers breadth; see traverse
1388, transeyn "crossbeam spanning an opening, lintel," probably by dissimilation from L. transtrum "crossbeam" (especially one spanning an opening), from trans- "across" + instrumental suffix -trum. Meaning "small window over a door or other window" is first recorded 1844.