Earlier, we discussed slot-boundp.  A named slot in an instance will not be bound to a value until it has been assigned by one of several means.  An unbound slot does not have a nil value, it has no value at all, and it is an error to attempt to read its contents.

Under some circumstances, the programmer may want to take a slot that has an associated value, and return it to that pristine, unbound state.  This is what slot-makunbound does.  If the slot has a bound value, that value is discarded and the slot is returned to the unbound state.