fundament

1. 1. The buttocks.
   2. The anus.
2. The natural features of a land surface unaltered by humans.
3. A foundation, as of a building.
4. An underlying theoretical basis or principle: "All neighbor states ... must revise ... their policy fundaments" (C.L. Sulzberger).

[Middle English foundement, from Old French fondement, from Latin fundāmentum, from fundāre, to lay the foundation, from fundus, bottom.]

noun

1. The lowest or supporting part or structure: base¹, basis, bed, bottom, foot, footing, foundation, ground, groundwork, seat, substratum, underpinning (often used in plural). See over/under.
2. That on which something immaterial, such as an argument or a charge, rests: base¹, basis, footing, foundation, ground (often used in plural), groundwork, underpinning (often used in plural). See over/under.
3. A fundamental principle or underlying concept: base¹, basis, cornerstone, foundation, fundamental, root¹, rudiment (often used in plural). See over/under.

1. a base or foundation, as the breech or rump.
2. the anus and parts adjacent to it.

The noun has 3 meanings:

Meaning #1: the fundamental assumptions underlying an explanation
  Synonyms: basis, base, foundation, groundwork, cornerstone

Meaning #2: the fleshy part of the human body that you sit on
  Synonyms: buttocks, nates, arse, butt, backside, bum, buns, can, hindquarters, hind end, keister, posterior, prat, rear, rear end, rump, stern, seat, tail, tail end, tooshie, tush, bottom, behind, derriere, fanny, ass

Meaning #3: lowest support of a structure
  Synonyms: foundation, base, foot, groundwork, substructure, understructure