Pronunciation /ˈsɒn(ə)rəs/ /səˈnɔːrəs/
(A voice or other sound) imposingly deep and full.
‘he read aloud with a xønørøuz and musical voice.’
Early 17th century from Latin sonorus (from sonor ‘sound’) + -ous.
The "Ø" possibly arose in Anglo-Saxon England as an O and an I written in the same place: compare Bede's Northumbrian Anglo-Saxon period spelling Coinualch for standard Cēnwealh (a man's name) (in a text in Latin). Later the letter ø disappeared from Anglo-Saxon as the Anglo-Saxon sound /ø/ changed to /e/, but by then use of the letter ø had spread from England to Scandinavia.
In Danish, ø is also a word, meaning "island".