I remember an episode from a game show. I am pretty sure it was "Wheel of Fortune." The task was to restore missing letters: an **ly child. The contestant answered "an ugly child", but the correct answer is "an only child." Moreover, I asked some people who know English well, and they replied that "an only child" is the only possible solution, though they didn't explain their choice in any way.
As for me, I consider the phrase "an only child" a little bit weird. "An ugly child" sounds nice for me, because he/she can be any of millions of such children, we just didn't specify the person we bear in mind. On the other hand, "only child" is a well defined person. There are no other children in the family; the child is single so we know whom we are talking about. So for me, it's like saying "a sun" or "a hell" or "an Eiffel Tower" which, as I know, are wrong constructions.
Can anyone explain why do we use indefinite article here? Does it have any particular or specific meaning?
Please don't blame it too much because in my native language there are no articles. Thanks for help.