firelock

n. 1. The lock of hair that grows from the forepart of the head. [ 1913 Webster ]

2. (Mech.) 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. [ 1913 Webster ]

Forelock bolt, a bolt retained by a key, gib, or cotter passing through a slot. --
Forelock hook (Rope Making), a winch or whirl by which a bunch of three yarns is twisted into a standard. Knight. --
To take time by the forelock or
To take occasion by the forelock, to make prompt use of anything; not to let slip an opportunity. [ 1913 Webster ]

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. [ 1913 Webster ]

On occasion's forelock watchful wait. Milton. [ 1913 Webster ]