Question on indefinite article (in couple weeks or in a couple weeks)

I have a question on indefinite articles. I thought a is only used with singular nouns, by definition.

Why is "in a couple weeks" the right way? Isn't a couple weeks more than one? I thought it would be "in couple weeks" but I see everywhere that it's used as "in a couple weeks". Can someone please be kind and explain why this is so?

In the idiom "a couple weeks" the indefinite article "a" modifies the singular noun "couple" according to the rule of indefinite articles.

Another example:

My wife and I are a couple.

As @John Lawler so aptly pointed out, the confusion is in the missing (yet still implied) preposition "of" that creates the illusion that "a" is modifying the plural "weeks". That would leave "couple" as an orphan in the phrase. Couple is not generally used as an adjective, and it couldn't correctly be used as an adjective in this case.

Idiom tends to defy grammar, but there is often a correct grammatical form supporting the idiom in its etymology.