Old English botm, bodan "ground, soil, foundation, lowest part," from Proto-Germanic *buthm- (cf. Old Frisian boden "soil," Old Norse botn, Dutch bodem, Old High German bodam, German Boden "ground, earth, soil"), from PIE root *bhu(n)d(h)- (cf. Sanskrit budhnah, Avestan buna- "bottom," Greek pythmen "foundation," Latin fundus "bottom, piece of land, farm," Old Irish bond "sole of the foot"). Meaning "posterior of a person" is from 1794. Bottom dollar "the last dollar one has" is from 1882. Bottom-feeder, originally of fishes, is from 1866.
1540s, "to put a bottom on," from bottom (n.). Meaning "to reach the bottom of" is from 1808 (earlier figuratively, 1785). Related: Bottomed; bottoming.
The buttocks; ass (1790s+)