What is "not worth a cress"

Cress \Cress\ (kr[e^]s), n.; pl. Cresses (kr[e^]s"[e^]z). [OE. ces, cresse, kers, kerse, AS. cresse, cerse; akin to D. kers, G. kresse, Dan. karse, Sw. krasse, and possibly also to OHG. chresan to creep.] (Bot.) A plant of various species, chiefly cruciferous. The leaves have a moderately pungent taste, and are used as a salad and antiscorbutic.

Note: The garden cress, called also peppergrass, is the Lepidium sativum; the water cress is the Nasturtium officinale. Various other plants are sometimes called cresses.

To strip the brook with mantling cresses spread.
--Goldsmith.

Bitter cress. See under Bitter.

Not worth a cress, or ``not worth a kers.'' a common old proverb, now turned into the meaningless ``not worth a curse.''
--Skeat.

a. worthless