What does "You'll do." mean?
I don't quite get it. I assume it's somehow related to love and relationships, and probably humorous. But what does it mean exactly? Where did it come from?

These are some different postcards:
1. You'll do. (heart-shaped)
2. Meh... You'll do♥ (tiny heart)
3. You're no Prince Charming but you'll do.
4. You'll do! It's a bit late to swap you now.

Thank you!

Question

What does "You'll do." mean?
I don't quite get it. I assume it's somehow related to love and relationships, and probably humorous. But what does it mean exactly? Where did it come from?

These are some different postcards:
1. You'll do. (heart-shaped)
2. Meh... You'll do♥ (tiny heart)
3. You're no Prince Charming but you'll do.
4. You'll do! It's a bit late to swap you now.

Thank you!

Answer

It means you're not perfect, but you're good enough. It is humorous in your examples, but it needn't be when applied to more practical situations:

The banana is a little too ripe, but it'll do for this recipe. The tire's tread is worn, but it'll do as a spare.

It probably originated as an abbreviation of "it will do the job," which means it is adequate to get the job accomplished.