Entry 1 sense Webster, 1913 nematocide /(nĕm"ȧ*tō̇*sīd [or] nĭ*măt"ō̇*sīd)/ · nem·a·to·cide · IPA 01 n. A substance that kills nematodes, especially one used in medicine or to kill plant pathogens. 1. A substance that kills nematodes, especially one used in medicine or to kill plant pathogens.