I have to confess I don't get The Doctor's reference. Am I too young? On the wrong continent?

So, three sailor song references to a long-tail blue (coat). It's adding up, on my calculator, to a fashion statement.

But The Doctor's excerpt from 'Cawsand Bay' begs the question, "What is dimity?' Anything like calico? Or chintz, p'raps? I'm just woefully undereducated on historical fabrics. Is there a reference out there?