It's just a euphemism, it's a sentence made funny by how much it understates the reality of the situation

That one's been around long enough I doubt anyone really knows its origin. It does follow a pattern of using the minute to describe various amounts of time.

For instance, a 'New York minute' means extremely quickly and a 'hot minute' is a long time, but much less than just 'it's been a minute'.

I guess it's the opposite of "I won't be a minute", which means I'll be fast.

Great flag, by the way.