The Collaborative International Dictionary of English v.0.48:

Pigweed \Pig"weed`\, n. (Bot.)
   A name of several annual weeds. See Goosefoot, and
   Lamb's-quarters.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pigweed
    n 1: common weedy European plant introduced into North America;
         often used as a potherb [syn: lamb's-quarters, pigweed,
         wild spinach, Chenopodium album]
    2: leaves sometimes used as potherbs; seeds used as cereal;
       southern United States to Central America; India and China
       [syn: pigweed, Amaranthus hypochondriacus]
    3: leaves collected from the wild [syn: lamb's-quarter,
       pigweed, wild spinach]