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