"Farmer's hat" is a term from the game of Go (Wei Qi/Badouk). It refers to the shape (closely related to the empty triangle) formed by the stones as shown below:

.....
..0..
.000.
.....

It is a bad shape for the same reason as the empty triangle; it has no eyes, few liberties and is overconcentrated. Note that it basically consists of two empty triangles. This illustrates the general principle that extensions of bad shape are also bad shape.