2. (Science: mechanics) a cotter or split pin, as in a slot in a bolt, to prevent retraction; a linchpin; a pin fastening the cap-square of a gun. Forelock bolt, a bolt retained by a key, gib, or cotter passing through a slot.
forelock hook, a winch or whirl by which a bunch of three yarns is twisted into a standard. To take time, or occasion, by the forelock, to make prompt use of anything; not to let slip an opportunity. Time is painted with a lock before and bald behind, signifying thereby that we must take time by the forelock; for when it is once past, there is no recalling it. (Swift) On occasion's forelock watchful wait. (Milton)