Yep, I had to look up this word as well. It is the name of the traditional 3 (tri) cornered (corn) hat worn during colonial times.
Thanks to the internet I was able to find easy instructions for a self-made tricorn hat made out of felt, fabric stiffener, glue gun, and a golden cord.
I wasn't quite sure if my fabric stiffener with an expiration date of 1995 (yeah, I know, one should throw things away but you see, I needed it, right?) would still work but with a generous application it stiffened up the felt nicely over night.
To give it some more flair we started curling some white curling ribbon to immitate the white wigs that were fashionable during that time as well.
Project cost: ~$4 and an hour of your time while having a good time with your child.