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

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.

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OXFORD DICTIONARY

forelock, n. a lock of hair growing just above the forehead.

Idiom

take time by the forelock seize an opportunity.

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