All things are a potential source of profit or advantage.

Grist is the corn that is brought to a mill to be ground into flour. In the days when farmers took ‘grist to the mill’ the phrase would have been used literally to denote produce that was a source of profit.

An early figurative use of phrase is found in Arthur Golding’s translation of The Sermons of J. Calvin upon Deuteronomie, 1583:

“There is no lykelihoode that those thinges will bring gryst to the mill.”