means The sudden, involuntary expulsion of air from the nose and mouth, triggered when your nasal passages decide something doesn't belong there.

from From Middle English 'snesen,' likely an alteration of the older 'fnesen' (to sneeze) — yes, words once started with 'fn.' That 'fn-' cluster traces back to Old English 'fnēosan' and has Germanic cousins like Dutch 'fniezen.' The shift from 'fn-' to 'sn-' may have been helped along by scribes misreading the rare 'f' as the long 's,' so the word essentially caught a typo and never recovered. The 'sn-' beginning, fittingly, clusters with a whole sniffly family: snore, snort, sniff, snot.