The Collaborative International Dictionary of English v.0.48:

Submarine \Sub`ma*rine"\, n.
   1. A submarine boat; a ship that can travel under the surface
      of the water. Most such ships are ships of war, as part of
      a navy, but submarines are also used for oceanic research.
      Also called sub and (from the German U-Boot) U-boat.
      esp., Nav., a submarine torpedo boat; -- called specif.
      submergible submarine when capable of operating at
      various depths and of traveling considerable distances
      under water, and submersible submarine when capable of
      being only partly submerged, i.e., so that the conning
      tower, etc., is still above water. The latter type and
      most of the former type are submerged as desired by
      regulating the amount of water admitted to the ballast
      tanks and sink on an even keel; some of the former type
      effect submersion while under way by means of horizontal
      rudders, in some cases also with admission of water to the
      ballast tanks.
      [Webster 1913 Suppl. +PJC]

   2. A stowaway on a seagoing vessel. [Colloq.]
      [PJC]

   3. A submarine sandwich.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

submarine sandwich \sub`ma*rine" sand"wich\, n.
   A large sandwich on an elongated roll, usually incompletely
   cut into two halves, filed with various cold cuts, meatballs,
   lettuce, cheese, tomatoes, olives, etc., and spiced
   variously, and often having oil or other dressing applied;
   called also hoagie, hero, hero sandwich, grinder,
   sub, submarine, poor boy, and Italian sandwich. A
   single such sandwich may consitute a substantial meal. Very
   large variants are sometimes prepared for social gatherings
   and cut into pieces for individual consumption.
   [PJC]

The Collaborative International Dictionary of English v.0.48:

Sub \Sub\, n.
   1. A subordinate; a subaltern. [Colloq.]
      [1913 Webster]

   2. a shortened form of submarine, the boat.
      [PJC]

   3. a shortened form of submarine sandwich; also called
      hero, hero sandwich, and grinder.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

Sub- \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-).
      [1913 Webster]

   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.]
      [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

114 Moby Thesaurus words for "sub":
   U-boat, U-boot, Unterseeboot, act for, agent, alternate,
   alternative, analogy, backup, change, change places with,
   changeling, collateral, common, comparison, copy, counterfeit,
   crowd out, cut out, demeaning, dependent, deputy, disadvantaged,
   displace, double, double for, dummy, equal, equivalent, ersatz,
   exchange, fake, fill in for, fill-in, ghost, ghostwrite,
   ghostwriter, humble, imitation, in the shade, inferior, infra dig,
   junior, less, lesser, locum tenens, low, lower, lowly, makeshift,
   metaphor, metonymy, minor, modest, next best thing, ordinary,
   personnel, phony, pigboat, pinch hitter, pinch-hit, proxy, relief,
   relieve, replace, replacement, represent, representative, reserves,
   ringer, second rank, second string, secondary, servile, sign,
   spares, spell, spell off, stand in for, stand-in, subaltern,
   subject, submarine, submersible, subordinate, subrogate,
   subservient, substituent, substitute, substitute for, substitution,
   succedaneum, succeed, supersede, superseder, supplant, supplanter,
   surrogate, swap places with, symbol, synecdoche, third rank,
   third string, token, tributary, under, underprivileged, understudy,
   understudy for, utility player, vicar, vice-president, vice-regent,
   vulgar