1.
to be the first to speak; to start things off; to break the ice
To be the one who initiates a discussion, debate, or action. Often used when someone is the first to speak up or take action in a group setting.
Originally a military term: 口火 refers to the fuse or primer of a firearm, so 口火を切る literally meant 'to light the fuse' or 'to open fire.' Now used figuratively to mean being the first to speak or act, triggering a chain of events.
Etymology
口火 is the opening of a furnace or the touch hole of a cannon — the small ignition point that sets off the main charge. The metaphor extends to being the catalyst that sets a discussion or movement in motion.