The Collaborative International Dictionary of English v.0.48:

Petty \Pet"ty\ (p[e^]t"t[y^]), a. [Compar. Pettier
   (p[e^]t"t[i^]*[~e]r); superl. Pettiest.] [OE. petit, F.
   petit; probably of Celtic origin, and akin to E. piece. Cf.
   Petit.]
   Little; trifling; inconsiderable; unimportant; also,
   inferior; subordinate; as, a petty fault; petty complaints; a
   petty prince. --Denham.
   [1913 Webster]

         Like a petty god
         I walked about, admired of all.          --Milton.
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   Petty averages. See under Average.

   Petty cash, money expended or received in small items or
      amounts.

   Petty officer, a subofficer in the navy, as a gunner, etc.,
      corresponding to a noncommissionned officer in the army.
      [1913 Webster]

   Note: For petty constable, petty jury, petty larceny, petty
         treason, See Petit.
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   Syn: Little; diminutive; inconsiderable; inferior; trifling;
        trivial; unimportant; frivolous.
        [1913 Webster]

WordNet (r) 3.0 (2006):

petty cash
    n 1: a small fund of cash that a firm keeps for the payment of
         incidental expenses