There is no widely accepted explanation for this old, but not exactly honorable, act of cutting class. However, it is clear that the term arose in 19th- century America, about the time that compulsory attendance became the rule in public schools.
Some word sleuths suggest that the word "hooky" may be a condensed version of the older expression "hook it," meaning to escape or make off.
Or it could be traced to the old term "hook," meaning to steal, as in a student stealing a day off from school.
"Hook" has so often been associated with going fishing, though, that it may simply be derived from the way a fish may wriggle its way off a hook
From the days of English knighthood came the expression "to buckle" oneself, meaning to fasten one's armor and prepare for battle. On the head, that meant to fasten the straps of the helmet securely in their holders along the cheek, originally called buccula in Latin. Later the word for this cheek clasp or hinge was extended to mean any belt fastener.
Figuratively, the expression "to buckle down" came to mean to apply oneself to a task, which may still include buckling oneself at the waist. ("Knuckle down" dates to the 1700s and the game of marbles. Shooters must have one knuckle of the hand on the ground for a shot to be legal.)