Tonic was also a popular term for any type of soda (coke, sprite, etc) in the northeast US, at least in the 1980's. And pronounced like tawnic.
My wife is from Boston too.
I love stuff like this, I've lived in Texas 13 years and have been pronouncing taco wrong until pointed out to me.
They say itmore like tocko than tacko like Brits do.
For me "tonic" has a fairly short vowel in the first syllable. Whereas "Tawnic" would have a longer vowel. There's also a different in sound. You might hear my "aw" as if it was a long "o" sound. The Australian vowel system is really nothing like the American vowel system.
The first syllable is drawn out much more than I would hear that word somewhere other than New England.
https://vocaroo.com/i/s1Se7U44m2c7
(I was going to go with Bostonian, Southie, but decided this one was better.)