Brian,
I believe it means that the parents aren't married and that the putative father has not come forward. In trying to follow them in later yrs, sometimes the parents (often one or both underaged) will marry; however, many times there is no subsequent marriage. For really early situations, Frances' Manx Note Book has some court issues of illegitimacy
Rarely, there is an explanatory or extra note in the parish record.

Donna