A knap-in, like a sit-in, love-in, etc., is a gathering of people to knap flint. Knap-ins often have the air of a medieval festival, with merchants selling arrowheads, rock collectors, atlatl contests, and so on.
Knappers often camp at the site of the knap-in, go hunting for flint nodules together, and of course, knap flint.
There are many knap-ins around the world; I went to one in Leavenworth, Indiana.
If you go to a knap-in, you may also have the chance to join Ooga-Booga Club.