DCHP-1 Online
double-jack [from its use in double-jacking, q.v.] Mining 
n. a heavy long-handled sledgehammer.
- 1944 double jack--an 8-pound striking hammer as used in hand drilling.
- 1961 The mechanization of mines in 1890--the replacing of hand-steel, single-jack and double-jack, by drilling machines--had created new problems and the mine owners had attempted to lower wages on the grounds that less skill and energy was required.
- 1944 double jack--an 8-pound striking hammer as used in hand drilling.