Source : Webster's Revised Unabridged Dictionary (1913)

Petty \Pet"ty\, a. [Compar. {Pettier}; superl. {Pettiest}.] [OE.
   petit, F. petit; probably of Celtic origin, and akin to E.
   piece. Cf. {Petit}.]
   Little; trifling; inconsiderable; also, inferior;
   subordinate; as, a petty fault; a petty prince. --Denham.

         Like a petty god I walked about, admired of all.
                                                  --Milton.

   {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.

   Note: For petty constable, petty jury, petty larceny, petty
         treason, See {Petit}.

   Syn: Little; diminutive; inconsiderable; inferior; trifling;
        trivial; unimportant; frivolous.

Source : WordNet®

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