The word ham denotes the part of the hindquarters of a pig or similar animal between the hock and the hip, hence, in cookery, the meat of this part, especially when salted or smoked.

The comparison between large hands and hams (aided by the alliteration ham–hand) gave rise to the adjectives ham-fisted and ham-handed, which mean:
– having large hands;
– hence also: heavy-handed, awkward, bungling.

These adjectives existed before the First World War. For instance, in Old Schenectady (New York – circa 1904), George Simon Roberts (born 1860) wrote:

Sir William had in his employ a large, ham-fisted Irishman named McCarthy, who was noted as the “champeen” bare-knuckle fighter of western New York.

And The Sporting Times (London) of 1^st June 1907 published The Gentle Art, a story by ‘Costs’, in which a character talks of a “hulkin’ ham-handed brute” who “slung [him] to roost among the minnows and the tittlebats [= sticklebacks]”.

But it was during the First World War that these adjectives gained currency in aeroplane pilots’ slang. The earliest use of ham-fisted applied to a pilot that I have found is from The Sketch (London) of 2^nd October 1918; in Humours of the air: some good stories of landing, C. G. Grey, editor of The Aeroplane, wrote:

The chief aim and object of every aviator worthy of the name is to put his aeroplane on the ground as gently as possible, and to leave it ‘planté là’, as it were, without rolling. The worst possible kind of landing, short of a real smash, is one which begins with a big bump, followed by a leap into the air, followed by a series of bounds along the ground, each smaller than the last, till the machine finally comes to rest. The stock jest of the lookers-on is to tell the ham-fisted author of such a landing that really he didn’t do badly, but that they thought his tenth or twelfth landing was the best of the lot.

The satirical magazine Punch, or the London Charivari of 3^rd April 1918 published the following fictional dialogue about aeroplane pilots’ slang, in which ham-handed, but also mutton-fisted, were used:

THE NEW LANGUAGE.
Scene.—R.F.C. [= Royal Flying Corps] Club.
Time.—Every Time.

1st Pilot. Why, it’s Brown-Jones!
2nd Pilot. Hullo, old thing! What are you doing now?
1st P. Oh, I’m down at Puddlemarsh teaching huns—monoavros, pups and dolphins.
2nd P. I’m on the same game, down at Mudbank—sop-two-seaters and camels. We’ve got an old tinside, too, for joy-riding.
1st P. You’ve given up the rumpety, then?
2nd P. Yes. I was getting ham-handed and mutton-fisted, flapping the old things every day; felt I wanted to stunt about a bit.
1st P. Have you ever butted up against Robinson-Smith at Mudbank? He was an ack-ee-o, but became a hun.
2nd P. Yes, he crashed a few days ago—on his first solo flip, taking off—tried to zoom, engine konked, bus stalled—sideslip—nose-dive. Not hurt, though. What’s become of Smith-Jones? Do you know?
1st P. Oh, yes. He’s on quirks and ack-ws. He tried spads, but got wind up. Have you seen the new ——?
2nd P. Yes, it’s a dud bus—only does seventy-five on the ceiling. Too much stagger, and prop stops on a spin. Besides I never did care for rotaries. Full of gadgets too.
1st P. Well, I must tootle off now. I’m flapping from Northbolt at dawn if my old airship’s ready—came down there with a konking engine—plug trouble.
2nd P. Well, cheerio, old thing—weather looks dud—you’re going to have it bumpy in the morning, if you’re on a pup.
1st P. Bye-bye, you cheery old bean.
                                                                                                              [Exeunt.