cacodyl

noun

1. the radical As(CH), composed of arsenic and methyl: its compounds are poisonous and foul-smelling
2. a poisonous, colorless liquid, As(CH), with an offensive odor: it is a polymer of this radical

Etymology: < Gr kakōdēs, bad-smelling < kakos, bad + -ōdēs (< IE base *od-, to smell > odor, ozone) + -yl