My reason doesn't make any sense and its horrible but here it goes:
When I was about 13 I still had my edgy twelvie handle "Nightmare", but one day as I was clicking through youtube
I came across a video of a woman screaming "WHO YOU FINN DAH TRY" before she got hit in the face with a bag of white powder.
No idea why but my brain sort of connected "FINN" and "DAH" and I made up Finnda for a new online handle because I thought it sounded nice at the time.
Friends at school started calling me Finn out of habit so it stuck.