To me, this is, always has been, and always will be a Captain Haddock insult.
You know Captain Haddock, from the Tintin books? He is given to very colourfully cussing out the various reprobates and recreants he and Tintin have to face in the adventures. Well, not cussing out. They’re kids’ books, after all! His shouted epithets for the fleeing villains include such wonders as bashi-bazouks, coelecanths, troglodytes, visigoths – look, there’s a whole list of 213 of them at www.tintinologist.org – and poltroons, which, if my memory is true, serves to set off a Himalayan avalanche.
Not all of these words are, in our real world, names for bad things or bad people. But a poltroon? A poltroon is just about the worst kind of person you’re likely to encounter. “A spiritless coward” and “a mean-spirited wretch,” Merriam-Webster Unabridged says; “an utter coward; a mean-spirited person; a worthless wretch,” the Oxford English Dictionary tells us. A poltroon is a soldier who shoots at his countrymen if they try to make him fight the enemy. A poltroon is a middle manager who won’t lift a finger to help his employees or advocate for them to upper management but will eagerly trap them in his office and belittle them at length. Captain Queeg, Herman Wouk tells us in The Caine Mutiny, is a poltroon.
But somehow, this term is not much used for full effect these days. As the OED says, it’s now chiefly archaic or humorous. For me, the Haddock effect is insuperable, but that can’t be the case for everyone. I think the echoes of goon, buffoon, baboon, loon, and such words probably have some effect. Echoes of dragoon, pantaloon, and saloon might add to the archaic feeling. And we have much more vulgar – and vividly metaphorical – terms to replace it, anyway.
What, originally, is a poltroon? It brings poultry to my mind, but that’s not quite what it is. A 17th-century author suggested that the word came from pollice truncus ‘maimed thumb’, referring to men who mutilated their thumbs to avoid military service. This was long accepted as the source, but it probably isn’t. Poltroon comes from Middle French poltron ‘coward’, from Italian poltrone ‘worthless person, coward’, tracing back to Latin pullus ‘young animal’ – which does connect closely to pullet and poultry, but they’re not the direct source of this word. But a poltroon is a chicken, so to speak – a chicken crossed with a jackal. Also an anthropophagus, an ostrogoth, a picanthropic pickpocket, and so much more…
Sesquiotica 
The perfect epithet for the would-be king of the white house
A fine word, sadly neglected. Over-40s rugby teams often have appropriate names: the Sundowners, the Ebb Tides, the Men o’ Paws (that was the Toronto Lions). And a team from Washington DC, the Poltroons. Jerseys royal blue, with a three-inch-wide yellow streak up the back. Mostly CIA spooks, I think. I remember the Poltroons fondly.
Scholar Salmasio’s ‘pollice truncus’ explanation seems too far fetched. My guess is from ‘poltrone’ (Italian adjetive for sluggard, bed ridden). From Italian ‘poltro’, and old-fashioned way to say ‘bed’, from Late Latin ‘pullitrus’ a young horse or bed frame, or torture tool.
Captain Herbert James Haddock was the first captain of the ill-fated Titanic – I wonder did this affect Herge’s selection of a name ? Tintin’s vituperative friend could just as readily have been called Captain Mackerel or Captain Cuttlefish … oh,wait, that was gone! Being Irish I had always presumed that “poltroon” had Gaelic origins. It sounds Gaelic. Thanks for the disillusionment!