From The Tommyknockers by Stephen King:
Late last night and the night before, Tommyknockers, Tommyknockers, knocking at the door. I want to run, don't know if I can, 'cause I'm so afraid of the Tommyknocker man.
The Knocker, Knacker, Bwca (Welsh), Bucca (Cornish) or Tommyknocker (US) is a mythical creature in Welsh, Cornish and Devon folklore.
-2. A spirit or goblin supposed to dwell in mines, and to indicate the presence of rich veins of ore by knocking.
The miners say the Knocker is some being that inhabits in the concaves and hollows of the Earth, and that it is thus kind to some men of suitable temper, and directs them to the ore by such this knocking.
Hooson, quoted by R. Hunt in British Mining.
But accounts may vary... The French version of the article discloses that the German Meister Hämmerlinge (maître marteleur) is also a Knocker. And King's Tommyknockers will tell you where to dig, yet their motives may be questionable.
Why is the word prefixed with "tommy" in AmE, where does it come from? Does it mean something different? The word tommy refers either to Thomas Atkins1, or to a diminutive of the name Thomas, a ration, or is related to the truck system i.e. the tommy would be the wager paid in goods instead of money. How is that related, if at all, to the knocker?
1. From the entry: [...] a familiar name for the typical private soldier in the British Army; arising out of the casual use of this name in the specimen forms given in the official regulations from 1815 onward. [...] Now more popularly Tommy Atkins or Tommy [...]. From wikipedia on the name Thomas: based on the Biblical Greek Θωμᾶς, which is itself a transcription of the Aramaic te'oma תאומא "twin", the Hebrew cognate being tə'ōm תאום.(Reference to citation being required, omitted.)