The Collaborative International Dictionary of English v.0.48:

Patronize \Pa"tron*ize\, v. t. [imp. & p. p. Patronized; p.
   pr. & vb. n. Patronizing.]
   1. To act as patron toward; to support; to countenance; to
      favor; to aid.
      [1913 Webster]

            The idea has been patronized by two States only.
                                                  --A. Hamilton.
      [1913 Webster]

   2. To trade with customarily; to frequent as a customer.
      [Commercial Cant]
      [1913 Webster]

   3. To assume the air of a patron, or of a superior and
      protector, toward; -- used in an unfavorable sense; as, to
      patronize one's equals.
      [1913 Webster]

WordNet (r) 3.0 (2006):

patronize
    v 1: assume sponsorship of [syn: sponsor, patronize,
         patronise]
    2: do one's shopping at; do business with; be a customer or
       client of [syn: patronize, patronise, shop, shop at,
       buy at, frequent, sponsor] [ant: boycott]
    3: treat condescendingly [syn: patronize, patronise,
       condescend]
    4: be a regular customer or client of; "We patronize this
       store"; "Our sponsor kept our art studio going for as long as
       he could" [syn: patronize, patronise, patronage,
       support, keep going]

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "patronize":
   aid, angel, assist, back, bankroll, boost, bridle, capitalize,
   condescend, contemn, contribute to, deal with, demean, disdain,
   do business with, encourage, finance, foster, frequent, fund,
   grubstake, have dealings with, have truck with, help, humiliate,
   pay for, promote, provide for, put down, refinance, scorn, set up,
   shop at, sponsor, stake, subscribe to, subsidize, support, take up,
   toss the head, trade at, trade with, traffic with,
   transact business with, treat, underwrite