An Etymological Dictionary of Astronomy and Astrophysics

1. The lowest or deepest part of anything, as distinguished from the → top. The under or lower side; underside.
   → bottom-up structure formation.
   

2. → bottom quark.

Etymology (EN): M.E. botme; O.E. botm, bodan “ground, soil, lowest part” (cf. O.Fris. boden “soil,” O.N. botn, O.H.G. bodam, Ger. Boden “ground, earth, soil”), akin to Pers. bon “basis; root; foundation; bottom;” Mid.Pers. bun “root; foundation; beginning;” Av. būna- “base, depth” (Skt. bundha-, budhná- “base, bottom,” Pali bunda- “root of tree;” Gk. pythmen “foundation;” L. fundus “bottom, piece of land, farm,” O.Ir. bond “sole of the foot”).

Etymology (PE): Tah “bottom; end”
(Mid.Pers. tah “bottom.” The origin of this term is not clear. It may be related to PIE *tenegos “water bottom;” cf. Gk. tenagos “bottom, swamp,” Latvian tigas, from *tingas, from
*tenegos “depth”).
Pâyin “bottom, below; at the foot of,” from pâ(y) “foot; step” (Mid.Pers. pâd, pây; Av. pad- “foot;” cf. Skt. pat; Gk. pos, genitive podos; L. pes, genitive pedis (Fr. pied); P.Gmc. *fot (E. foot; Ger. Fuss);
PIE *pod-/*ped-); + -in a relation suffix.