What a splendid thread !! Thankyou all, for evoking those images of the great Duncan MacCrae, as Para Handy , nonchalantly leaning over the voice-pipe of the "Vital Spark " and asking, in his inimitable Highland lilt, " Wooed you be geefing me fool ahead, just now ? "
    Personally, I think that " Blow the man down" is a straightforward corruption. By the days of the great Tea Clippers, like the " Tie Pin", the " Thermocouple", the " Antenna " ( or was it " Aerial " ?) and of course , the "Sooty Shark " ( which is certainly sooty now ), the sailors had become bored with engraving pornographic images onto bits of whalebone,as started by a man called Grimshaw, and were casting around for something else with which to fill their idle hours.As it happens, at about that time, these great ships were carrying numbers of Chinamen in their crews, and someone hit on the idea of learning Chinese from them, so that the crew could converse, and ,indeed, make derogatory remarks of a personal and sensitive nature about the Captain and the Bosuns Mates, without being understood. All went well for a while, but inevitably ,the Masters and Mates got wind ( no comedians, please ) of this , and thus it soon became customary aboard the Clippers, whenever Chinese was heard being spoken, for the Bosun"s Mate to whip out his call, pipe the shrill, and bellow " BELAY THE MANDARIN ! ". Quod Erat Demonstrandum , eh ?? I mean, if that isn"t a plausible explanation, then what is ???