to rush into action, to be eager to do something · to do something without proper consideration
köken: A stapel is a set of supporting structures that hold together a ship while it is being built in a shipyard. Hence, the whole expression refers to rushing a ship to the water.
to jump the gun (to do something earlier than it was supposed to be done, without discussing it with other people with whom one forms a group or organization)
köken: Literally, “to come out in front of a line”.