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]