1. [n]  a submersible warship usually armed with torpedoes
2. [n]  a large sandwich made of a long crusty roll split lengthwise and filled with meats and cheese (and tomato and onion and lettuce and condiments); different names are used in different sections of the United States
3. [v]  be a substitute

1. \Sub-\ [L. sub under, below; akin to Gr. ?, Skr. upa to,
   on, under, over. Cf. {Hypo-}, {Super-}.]
   1. 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-).
   2. (Chem.) 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. [Obsoles.]
   

2. \Sub\, n.
   A subordinate; a subaltern. [Colloq.]
   

Substitute