The word 'sneeze' was originally 'fneeze' and is related to Old Norse fnaesa = snort or sneeze. But it lost its initial 'f', and became 'nese', probably because 'fn' is such a rare word beginning in English (I can't find any examples in modern English). But later it acquired an 's'. Perhaps people thought the 'f' was a long s, ƒneeze, or possibly as the Oxford English Dictionary suggests adding an initial 's' to 'nese' made it sound sneezier. The word 'cough' sounds like a cough (it's echoic) and 'sneeze' sounds like a sneeze.
Going further back, 'fneeze, is probably from the same older root as Greek πνευ (pneu) = breath, as in pneumonia.
This change from p to f, going from Greek/Latin to Germanic languages, is a regular feature, first articulated by Schlegel in 1806. Other examples are pyr->fire, pod->foot, pater->father, penta->five, pulo->fowl, and (more surprisingly) port -> ford. (So a ford is a gateway/door (or port)).
See here for more discussion about this sound change, part of Grimm's (one of the fairy tale brothers) law.
Coughes and Fneezes
Neyther mayst thou ryse, cough, spyt, or nese
A. Barclay, Egloges, ca. 1530
The word 'sneeze' was originally 'fneeze' and is related to Old Norse fnaesa = snort or sneeze. But it lost its initial 'f', and became 'nese', probably because 'fn' is such a rare word beginning in English (I can't find any examples in modern English). But later it acquired an 's'. Perhaps people thought the 'f' was a long s, ƒneeze, or possibly as the Oxford English Dictionary suggests adding an initial 's' to 'nese' made it sound sneezier. The word 'cough' sounds like a cough (it's echoic) and 'sneeze' sounds like a sneeze.
Going further back, 'fneeze, is probably from the same older root as Greek πνευ (pneu) = breath, as in pneumonia.
This change from p to f, going from Greek/Latin to Germanic languages, is a regular feature, first articulated by Schlegel in 1806. Other examples are pyr->fire, pod->foot, pater->father, penta->five, pulo->fowl, and (more surprisingly) port -> ford. (So a ford is a gateway/door (or port)).
See here for more discussion about this sound change, part of Grimm's (one of the fairy tale brothers) law.