Luckily, a friend of mine has a cousin called Nienke! This is what she gave me:
you believe it, my real names are Trijntje
Bootsma (my Frisian
grandmother's first name and maiden name). But according to my Frisian
father the nickname (noemnaam) for Trijntje
is Nienke - not sure why, but I know another Frisian Nienke who's real name is also Trijntje
. I think it is quite possible that Trijntje
comes from Catharina
and then became Nienke."
So, there we have it! She's right about Trijntje
coming from Catharina
- it would be pronounced more-or-less like train-ke - and, I think, confirms my suspicion.
All the best