transom (n.) Look up transom at Dictionary.com
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.