Synonyms and related words :
bed, bedrock, belly, below, beneath, bottom, bottom side, breech, buttocks, clandestine, covert, downside, fundament, furtive, hardpan, hole-and-corner, lower side, lowest layer, lowest level, neath, nether side, nethermost level, rock bottom, sole, stealthy, subordinate to, substratum, surreptitious, under, under-the-table, underbelly, undercover, underlayer, underside