Learn English – “Who him?” as a stand-in for “Who is he?”

I'm curious: is this valid under some rule of grammar I don't know? Was it ever valid, or was it slang or a personal idiosyncrasy? Or (I shudder to think) was it invented by later authors, as a phrase that sounds sufficiently outlandish to add "flavor" to a novel?

I've run across this construction in several "period" novels set in the 18th century, most recently in The Yellow Admiral by Patrick O'Brian:

'I shall just have time to run up to
the House for the committee meeting,
deliver my thunderbolt, and then post
down to Torbay, where Heneage Dundas
will touch before the change of the
moon, landing Jenkins -,

'Who him?'

'My jobbing captain, my temporary
replacement,' said Jack, and from his
tone and the set of his face Stephen
gathered that he did not think highly
of the man.

I'd like to clarify: I am not talking about "Who, him?" ("are you seriously talking about THAT guy?"), nor am I talking about the poetic reversal of verb and object, as in Spenser's Faerie Queen:

At last, when as himself he 'gan to find
To Una back he cast him to retire:
Who him awaited still with pensive Mind.
Great thanks and goodly Meed, to that good Sire,
He thence departing gave for his pains Hire.
So came to Una, who him joy'd to see,
And after little rest, 'gan him desire,
Of her Adventure mindful for to be.
So leave they take of Caelia, and her Daughters three.

I ask because… well, I rather like the sound of it, and have been tempted to slip it into my own conversation. But I'd like to have cover for the inevitable discussion afterward!