Sub-

CIDE DICTIONARY

Sub-,  [L. sub under, below; akin to Gr. , Skr. upa to, on, under, over. Cf. Hypo-, Super-.].

•  A prefix signifying under, below, beneath, and hence often, in an inferior position or degree, in an imperfect or partial state, as in subscribe, substruct, subserve, subject, subordinate, subacid, subastringent, subgranular, suborn. Sub- in Latin compounds often becomes sum- before m, sur before r, and regularly becomes suc-, suf-, sug-, and sup- before c, f, g, and p respectively. Before c, p, and t it sometimes takes form sus- (by the dropping of b from a collateral form, subs-).  [1913 Webster]
•  A prefix denoting that the ingredient (of a compound) signified by the term to which it is prefixed,is present in only a small proportion, or less than the normal amount; as, subsulphide, suboxide, etc. Prefixed to the name of a salt it is equivalent to basic; as, subacetate or basic acetate.  [1913 Webster]

OXFORD DICTIONARY

Sub-, prefix (also suc- before c, suf- before f, sug- before g, sup- before p, sur- before r, sus- before c, p, t)
1 at or to or from a lower position (subordinate; submerge; subtract; subsoil).
2 secondary or inferior in rank or position (subclass; subcommittee; sub-lieutenant; subtotal).
3 somewhat, nearly; more or less (subacid; subarctic; subaquatic).
4 (forming verbs) denoting secondary action (subdivide; sublet).
5 denoting support (subvention).
6 Chem. (of a salt) basic (subacetate).

Etymology

from or after L sub- f. sub under, close to, towards