Words at Play : Monster of the Day
"The 'graveyard' shift is the dead of night usually between 11 p. m. and 3 a. m. and it is then that the 'tommy-knockers' are most often heard." Mexico Missouri Message (Mexico, MO), August 23, 1906
Definition:
: the ghost of a man killed in a mine
Word Origin:
Probably from Tommy (nickname for Thomas) + knocker; from his being supposed to be responsible for the creaking of timbers in the mine
Photo credit: Tory Novikova