Struggling a bit, but 'nic' is the feminine of 'mac' and means 'daughter of'.
Forfarian, you are right, if it were normative Scots Gaelic 'nic' would be daughter of. But the transcriptions in question seem to be Hew Morrison's heavy editing of Rev. Thomson's unique on-the-fly-and-meandering transliterations of Scots Gaelic, hence I discard normative spelling rules to make a best guess.
For context my 4x GGF Hugh Ross (1731-1799 + buried at Balnakeil) brought infants to Rev. Thomson for baptism in '66, '68, '71, '75, '78, and '81, and the result was a similar set of meandering and creative variations that collectively kind of approximate his patronymic.
Fiveraliver, I read the groom you describe as "son of John son of John son of George son of William" and I'm not certain how to read the bride. However, I'm concerned that the four baptisms you mention are not all the same parents, i.e., the married couple Hugh-Mackay+Isobel-Skerray-Mackay. For example, Angi Lamb did not explicitly merge together the four Hugh+Isobel baptism entries in your post as a single couple in her "Index of Sutherland Durness Baptisms and Births 1764-1874." Let me know if you don't have Angi's document - it's posted in several places, such as in the Sutherland County Family History group on Facebook. Or you can PM me your email and I'll send it to you.