means To expel air, mucus, and irritants suddenly and involuntarily through the nose and mouth, usually in response to irritation of the nasal lining.

from From Middle English 'snesen' or 'fnesen,' going back to Old English 'fnēosan,' to sneeze — that 'fn-' cluster (a cousin of words across the Germanic family) was meant to sound like the act itself. Over time the awkward 'fn-' likely got misread or reshaped into 'sn-', possibly nudged along by the related 'snore' and 'snort,' which also wear that nasal 'sn-' on their sleeve. The whole family is essentially the language imitating a nose doing something noisy.