When a student is unprepared for a class, either has not done the homework assignment or did not do the work, or maybe even forget the materials needed for the class and as a result, the student tries to lay low the entire class and not get called on by the teacher.
Flat-fishers may be seen averting eye contact with the teacher, pretending to be very focused on something in their notebook, or pretending to take notes. Overall, just trying to go unnoticed until the class is over.
I didn't do the homework. I plan on flatfishing the entire class.