An onecht kind is a child born out of wedlock. You may encounter the term in baptismal records or court records.
Here are tips for finding the father of an illegitimate child.

Baptism of a child, 1956. Credits: Willem van de Poll, collection Nationaal Archief (CC-BY)
Just over a week ago after many years of searching we found my husbands Great Grandfather.
His Great Grandfather was living with his Great Grandmother and were found in the population registers