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transom (n.)

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.