What is "to take occasion by the forelock"

Forelock \Fore"lock`\, n.

1. The lock of hair that grows from the forepart of the head.

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.

   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.

   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.