Old English fneosan "to snort, sneeze," from Proto-Germanic *fneusanan (cf. Middle Dutch fniesen, Dutch fniezen "to sneeze;" Old Norse fnysa "to snort;" Old Norse hnjosa, Swedish nysa "to sneeze;" Old High German niosan, German niesen "to sneeze"), from Proto-Germanic base *fneu-s- "sneeze," of imitative origin, as is PIE *pneu- "to breathe" (cf. Greek pnein "to breathe").
Other imitative words for it, perhaps in various ways related to each other, include Latin sternuere (cf. Italian starnutare, French éternuer, Spanish estornudar), Breton strevia, Sanskrit ksu-, Lithuanian čiaudeti, Polish kichać, Russian čichat'. English forms in sn- appear late 15c.; change may be due to a misreading of fn-, or from Norse influence. But OED suggests Middle English fnese had been reduced to simple nese by early 15c., and sneeze is a "strengthened form" of this, "assisted by its phonetic appropriateness." Related: Sneezed; sneezing. To sneeze at "to regard as of little value" (usually with negative) is attested from 1806.