The Collaborative International Dictionary of English v.0.48:

Penny \Pen*ny\, n.; pl. Penniesor Pence (p[e^]ns). Pennies
   denotes the number of coins; pence the amount of pennies in
   value. [OE. peni, AS. penig, pening, pending; akin to D.
   penning, OHG. pfenning, pfenting, G. pfennig, Icel. penningr;
   of uncertain origin.]
   1. A former English coin, originally of copper, then of
      bronze, the twelfth part of an English shilling in account
      value, and equal to four farthings, or about two cents; --
      usually indicated by the abbreviation d. (the initial of
      denarius).
      [1913 Webster]

   Note: "The chief Anglo-Saxon coin, and for a long period the
         only one, corresponded to the denarius of the Continent
         . . . [and was] called penny, denarius, or denier."
         --R. S. Poole. The ancient silver penny was worth about
         three pence sterling (see Pennyweight). The old
         Scotch penny was only one twelfth the value of the
         English coin. In the United States the word penny is
         popularly used for cent.
         [1913 Webster]

   2. Any small sum or coin; a groat; a stiver. --Shak.
      [1913 Webster]

   3. Money, in general; as, to turn an honest penny.
      [1913 Webster]

            What penny hath Rome borne,
            What men provided, what munition sent? --Shak.
      [1913 Webster]

   4. (Script.) See Denarius.
      [1913 Webster]

   Penny cress (Bot.), an annual herb of the Mustard family,
      having round, flat pods like silver pennies (Thlaspi
      arvense). Also spelled pennycress. --Dr. Prior.

   Penny dog (Zool.), a kind of shark found on the South coast
      of Britain: the tope.

   Penny pincher, Penny father, a penurious person; a miser;
      a niggard. The latter phrase is now obsolete. --Robinson
      (More's Utopia).

   Penny grass (Bot.), pennyroyal. [R.]

   Penny post, a post carrying a letter for a penny; also, a
      mail carrier.

   Penny wise, wise or prudent only in small matters; saving
      small sums while losing larger; penny-wise; -- used
      chiefly in the phrase, penny wise and pound foolish.
      [1913 Webster]

WordNet (r) 3.0 (2006):

penny grass
    n 1: foetid Eurasian weed having round flat pods; naturalized
         throughout North America [syn: field pennycress, French
         weed, fanweed, penny grass, stinkweed, mithridate
         mustard, Thlaspi arvense]