noun
The lowest part
underside, nether portion, base, nadir, foot, pediment, depths, bed, floor, lowest part, deepest part, sole, ground, underpart.
Antonyms top*, peak, pinnacle.
The foundation
base, substructure, support; see foundation 2.
The source
cause, basis, origin, root; see origin 2, 3.
*The buttocks
behind, seat, rear; see back 2, rump.
at bottom
be at the bottom of
bet one's bottom dollar*
bet everything one has, be certain, be assured; see bet, trust 1.