WordNet (r) 3.0 (2006):

rock bottom
    n 1: the absolute bottom

Moby Thesaurus II by Grady Ward, 1.0:

67 Moby Thesaurus words for "rock bottom":
   base, basement, basis, bearing wall, bed, bedding, bedrock, belly,
   best, bottom, bottom side, bottommost, breech, buttocks, cut,
   deepest, downside, floor, flooring, fond, footing, foundation,
   fundament, fundamental, giveaway, ground, grounds, groundwork,
   half-price, hardpan, lower side, lowest, lowest layer,
   lowest level, marked down, nether side, nethermost,
   nethermost level, pavement, principle, radical, reduced, riprap,
   rudiment, sacrificial, seat, sill, slashed, solid ground,
   solid rock, stereobate, stylobate, substratum, substruction,
   substructure, terra firma, underbelly, underbuilding,
   undercarriage, undergirding, underlayer, undermost, underneath,
   underpinning, underside, understruction, understructure