WordNet (r) 3.0 (2006):

weed killer
    n 1: a chemical agent that destroys plants or inhibits their
         growth [syn: herbicide, weedkiller, weed killer]

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "weed killer":
   acaricide, anthelmintic, antibiotic, antiseptic, bug bomb,
   carbamate insecticide, chemosterilant,
   chlorinated hydrocarbon insecticide, contact poison, defoliant,
   disinfectant, eradicant, fumigant, fungicide, germicide, herbicide,
   insect powder, insecticide, microbicide, miticide,
   organic chlorine, organic phosphate insecticide, pesticide, poison,
   rat poison, roach paste, roach powder, rodenticide, stomach poison,
   systemic, systemic insecticide, toxic, toxicant, toxin, venin,
   venom, vermicide, virus