A thunk, as used here, is a provisional chunk of thought. A thunk is somewhere between a tweet and a blog post. It may or may not be very good or clear or even coherent. But, it is, nevertheless, worth making public.
Why? Three reasons really: firstly, to stem the tendency to procrastinate and leave good ideas in notebooks or in one’s head; secondly, to log and track ideas over time; and thirdly, to solicit feedback and comment with a view to sharpening (or revising) the idea.