strike while the iron is hot

Meanings

1. To strike a hot piece of metal, especially iron, with a mallet or other tool before it cools, while it is still hot enough to be shaped.
2. To act on an opportunity promptly while favorable conditions exist; to avoid waiting.

Etymology

The adage originates from the common craft of blacksmithing. It appears in Chaucer's The Tale of Melibee, "right so as whil that iren is hoot men sholden smyte". (Harvard) It gained additional meaning when used as a motto by Joseph Paisley - a blacksmith at Gretna Green who married eloping couples at his anvil. (History in the Margins)