What is the connection between frown, nose and nostrils?

The English word frown comes from the Middle English frounen (to frown as an expression of disapproval, displeasure, shame, fear, or jealousy), from the Old French frognier (to frown or scowl), from Gaulish *frognā (nostril), from the Proto-Celtic *srognā, from the Proto-Indo-European *sregʰ- (snore) [source].

The Proto-Celtic word *srognā is the root of the following words in the modern Celtic languages:

• Irish (Gaeilge): srón [sˠɾˠoːnˠ] = nose; sense of smell; prow, projection
• Scottish Gaelic (Gàidhlig): sròn [sdrɔːn] = nose, snout, trunk; promontory; snout (of a glacier); toe (of a shoe)
• Manx (Gaelg): stroin [strɛin] = nose, promontory, headland, ness, naze, nose-piece
• Welsh (Cymraeg): ffroen = nostril; muzzle of a gun, mouth of a cannon, nozzle of a pair of bellows; hole, entrance, opening (of a pipe), spout
• Cornish (Kernewek): frig [fri:g] = nostril
• Breton (Brezhoneg): froen = nostril, fri = nose

I’m not sure if the Cornish word frig comes from the same root, but it seems likely.

The French word renfrogner (to scowl), the Galician word enfurruñar (to frown, to get angry), the Spanish word enfurruñarse (to get angry, get cross, to sulk, to cloud over) also come from the same root.

Sources: Wiktionary, Am Faclair Beag, Online Manx Dictionary, Teanglann.ie, Geiriadur Prifysgol Cymru, Gerlyver Kernewek, Dictionnaire Favereau, Reverso

This is one of the connections I found recently while working on the Celtiadur, my collection of Celtic cognates.