mopboard†; bedrock, hardpan [U.S.]; foundation &c (support) 215; substructure, substratum, ground, earth, pavement, floor, paving, flag, carped, ground floor, deck; footing, ground work, basis; hold, bilge. bottom, nadir, foot, sole, toe, hoof, keel, root; centerboard. Adj. bottom, undermost, nethermost; fundamental; founded on, based on, grounded on, built on.