trigger

What it means

1. engineeringA finger-operated lever used to fire a gun.

   Just pull the trigger.

   1 more example

   [W]e pull'd aſide the Tricker, and obſerv'd, that according to our expectation the force of the Spring of the Lock vvas not ſenſibly abated by the abſence of the Air.

   1659 December 30 (date written), Robert Boyle, “[Experiment 14]”, in New Experiments Physico-Mechanicall, Touching the Spring of the Air, and Its Effects […], Oxford, Oxfordshire: […] H[enry] Hall, printer to the University, for Tho[mas] Robinson, published 1660, →OCLC, page 89

2. A similar device used to activate any mechanism.
3. An event that initiates others, or incites a response.

   Sleeping in an unfamiliar room can be a trigger for sleepwalking.

In a sentence

Just pull the trigger.

Where it comes from

1. Dutchtrekken“to drag, draw, pull”
2. Dutchtrekker“pull”
3. Englishtrigger

Originally tricker, from Dutch trekker (“pull”, noun, as in drawer-pull, bell-pull), from Dutch trekken (“to drag, draw, pull”).