"A jack" meant a worker. As in "jack-of-all-trades."

"Now there was Billy Anderson,
A jack from Puget Sound,
A fella who could lift a ton
Like some men lift a pound."
-- _Tote-Road And Trail, Ballads of The Lumberjack_, D. Malloch, 1917.

Some workers in the mountains hollered loud, e.g., "Timber."

Sid Harris (1885-1927), a pianist on the T.O.B.A. circuit in Alabama, wrote the song "Mountain Jack Blues," 1926, which was recorded by Ma Rainey, who was well-known and widely copied.