Origins
The word 'smell' is one of the more etymologically mysterious words in English core vocabulary. It appears in Middle English around 1175 as 'smellen,' meaning both 'to emit an odour' and 'to perceive an odour,' but its earlier history is frustratingly obscure. No cognate form is attested in Old English — the word seems to emerge fully formed in the twelfth century, suggesting either an unrecorded Old English ancestor (*smyllan or *smellan) or a borrowing from a related Low Germanic language.
The most widely accepted theory connects 'smell' to Middle Low German 'smelen' and Middle Dutch 'smölen' (to smoulder, to burn slowly). If this connection is correct, the semantic path is evocative: the original concept was not 'to perceive with the nose' but 'to smoulder, to emit slowly' — like smoke rising from embers. Smell was conceived as something that emanates, that drifts invisibly through the air, like the fumes from a slow fire. The shift from 'to smoulder/emit' to 'to smell' would represent a metonymic transfer from the source to the sense that perceives it.
Before 'smell' arrived, Old English had several words for olfactory perception. The most common was 'stincan,' which originally meant simply 'to smell, to emit an odour' — entirely neutral, applicable to roses as well as rot. Its German cognate 'stinken' underwent the same narrowing to 'to stink,' while the original neutral sense was filled by 'riechen' in German. English went through an analogous replacement: 'smell' took over as the neutral term, and 'stink' was demoted to negative-only duty. This pattern — a neutral word acquires negative connotations, a new neutral term replaces it, and the old word is left with only the pejorative sense — is a well-documented process in historical linguistics called pejoration.
Proto-Indo-European Roots
The word 'smell' is unusual among sense-words in having no clear PIE pedigree. 'See' goes back to PIE *sekʷ- (to follow, to perceive). 'Hear' connects to PIE *h₂ḱous- (to hear). But 'smell' has no confident reconstruction beyond Proto-Germanic, making it one of the youngest and most opaque of the English sense-words.
The noun use of 'smell' (an odour, a scent) dates from the thirteenth century. 'Smelly' (having a bad smell) appeared in the mid-nineteenth century. The metaphorical use — 'I smell trouble,' 'something smells fishy' — treats the nose as a detector of hidden truth, echoing the ancient belief that bad smells accompanied moral corruption and evil.
English is unusual among European languages in having a single word ('smell') that is both neutral in valence and covers both the active sense (to sniff) and the passive sense (to emit an odour). French separates 'sentir' (to smell something) from 'puer' (to stink) and 'embaumer' (to smell sweet). German separates 'riechen' (to smell, neutral) from 'stinken' (to stink) and 'duften' (to smell pleasant). English 'smell' does all this work alone, modified by context and adjectives.