Slaught-beam

Probably a calque of Dutch slagboom or German Schlagbaum. These are etymologically equivalent to English slay + beam, meaning a beam that can be banged (“slain”) down. The word slaught (related with slay) would have been chosen because it is phonetically closer to the continental words. Alternatively it might be a phono-semantic matching of Dutch sluitboom, which has the same sense, but literally means “closing beam”. In this case, however, the etymological relation between the element slaught and the above-mentioned words would be a curious coincidence.