Source : Webster's Revised Unabridged Dictionary (1913)

Lethiferous \Le*thif"er*ous\ (l[-e]*th[i^]f"[~e]r*[u^]s), a. [L.
   lethifer, letifer, fr. lethum, letum, death + ferre to bear,
   to bring: cf. F. l['e]thif[`e]re.]
   Deadly; bringing death or destruction.