A joist is literally "the thing on which everything lies" — sharing its root with gist (where a case rests) and gîte (a place to rest in the French countryside).
A horizontal supporting beam in a floor or ceiling, to which boards or laths are nailed; one of a series of parallel beams that carry the weight of a floor or roof.
From Old French giste (beam supporting a bridge, a resting place), from gesir (to lie, to rest), from Latin iacēre (to lie, to rest), from PIE *yeh₁- (to throw, to lie). The beam is named because it is the thing on which the floor lies or rests. Key roots: *yeh₁- (Proto-Indo-European: "to throw, to lie").
The joist shares its root with the French gîte (a holiday cottage or rural lodging) — both come from Old French gesir (to lie, to rest). A joist is literally the thing on which everything else lies. The same Latin root iacēre (to lie) that produces joist also produces adjacent (lying near) and the gist of an argument (the place where a legal case lies or rests). So the structural beam under your floor