(intransitive, of a horse or similar saddle or pack animal) To leap upward arching its back, coming down with head low and forelegs stiff, forcefully kicking its hind legs upward, often in an attempt to dislodge or throw a rider or pack.
(transitive, of a horse or similar saddle or pack animal) To throw (a rider or pack) by bucking.
(transitive, military) To subject to a mode of punishment which consists of tying the wrists together, passing the arms over the bent knees, and putting a stick across the arms and in the angle formed by the knees.
(intransitive, by extension) To resist obstinately; oppose or object strongly.
(intransitive, by extension) To move or operate in a sharp, jerking, or uneven manner.
(transitive, by extension) To overcome or shed (e.g., an impediment or expectation), in pursuit of a goal; to force a way through despite (an obstacle); to resist or proceed against.
(riveting) To press a reinforcing device (bucking bar) against (the force of a rivet) in order to absorb vibration and increase expansion. See Wikipedia: Rivet:Installation.
(forestry) To saw a felled tree into shorter lengths, as for firewood.
(electronics) To output a voltage that is lower than the input voltage. See Wikipedia: Buck converter