From Latin 'fundus' (bottom, ground) — kin to 'fund,' 'fundamental,' and 'profound,' all built on what lies at the bottom.
The lowest load-bearing part of a building, typically below ground level; more broadly, the basis or groundwork of anything; also an institution established with an endowment for charitable or educational purposes.
From Old French 'fondacion,' from Latin 'fundātiōnem' (a founding, foundation), from 'fundāre' (to lay the bottom, to found, to establish), from 'fundus' (bottom, base, piece of land, estate). The etymological chain is concrete and spatial: the 'fundus' is the bottom — the lowest point, the ground itself — and to 'found' something is to place it on that bottom, to give it a base. All the modern senses — architectural, institutional, and abstract — flow from this single metaphor: what lies at the bottom supports everything above it.
The words 'foundation,' 'fund,' 'fundamental,' and 'profound' all trace back to Latin 'fundus' (bottom). A charitable 'fund' is money set at the bottom as a base; something 'profound' is 'from the bottom' (pro + fundus); and 'fundamental' means 'pertaining to the bottom.' English built an entire vocabulary of depth and importance on one Latin word for 'ground.'