US Military Slang meaning: To recommend, suggest, or order an action to be quickly or immediately taken.
Originates from sending orders or information down the
line. A soldier would tell another soldier
something, and say "Send it (down the line)", causing the other soldier to do the same to the next soldier, until the message is completely disseminated along the line.
The term developed independently in the civilian rock climbing community to
mean the same thing. Has recently become part of civilian slang, probably originating here.
Also used informally by tankers or redlegs to
mean fire the main
gun or howitzer, to send a round downrange. Later used for firing any projectile from any weapon system.