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-).