means To provide or make available something that is wanted or needed; or, as a noun, the amount of a thing on hand to be had.

from From Latin 'supplere,' to fill up or complete — built from 'sub-' (up from below) and 'plere' (to fill), making it a cousin of 'plenty,' 'complete,' and 'replenish.' It reached English through Old French 'soupleer,' carrying that original sense of filling a gap. The economic meaning — stock available to a market — is a later, more commercial settling of the same idea: you supply by filling a need.