Hello Margaret,
I was so interested to see this post. I have wondered about this entry in the IGI also.
I think what it means is that the father of this child may have been a natural child of a Morison or a Nobleman (Nobleman as a name rather than an occupation) and either Morison or Nobleman could be his father's name and vice versa for his mother.
The entry that I have seen refers to a William Morison born 5th Jan 1767 in Alva, Stirling. I haven't seen one in 1789???
The reason that I am interested in it is because having come to a complete standstill with my Andrew Morison married Margaret Baird, Falkirk 1800, I am now relying on the extremely scientific method

of good old Scottish Naming Patterns to try and find some way round my brickwall.
The children's names of Andrew and Margaret suggest that his parents were William and Christian and there are not so many with this combination, although I haven't found that this particular couple - William Morison and Christian Dempster (sometimes Demperston according to LDS submission) had a son Andrew .......
Can I ask how he fits in your family? I think I had a email from someone once who was connected to this family, I'll have a look and ask her to have a look at your post also.
regards,
Lindsay