a small projecting tongue in a firearm that, when pressed by the finger, actuates the mechanism that discharges the weapon.
a device, as a lever, the pulling or pressing of which releases a detent or spring.
anything, as an act or event, that serves as a stimulus and initiates or precipitates a reaction or series of reactions.
Slang. triggerman.
verb (used with object)
to initiate or precipitate (a chain of events, scientific reaction, psychological process, etc.): Their small protest triggered a mass demonstration.
to fire or explode (a gun, missile, etc.) by pulling a trigger or releasing a triggering device: He accidentally triggered his rifle.
verb (used without object)
to release a trigger.
to become active; activate.
quick on the trigger, Informal. quick to act or respond; impetuous; alert.

1615–25; earlier tricker < Dutch trekker, equivalent to trekk(en) to pull + -er -er1

untriggered, adjective Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
trigger (ˈtrɪɡə)
1.  a small projecting lever that activates the firing mechanism of a firearm
2.  machinery a device that releases a spring-loaded mechanism or a similar arrangement
3.  any event that sets a course of action in motion
4.  (usually foll by off) to give rise (to); set off
5.  to fire or set in motion by or as by pulling a trigger
[C17 tricker, from Dutch trekker, from trekken to pull; see trek]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

1660, earlier tricker (1621), from Du. trekker "trigger," from trekken "to pull" (see trek). Tricker was the usual form in Eng. until c.1750. The verb is first recorded 1930, from the noun. Trigger-happy is attested from 1943.
Online Etymology Dictionary, © 2010 Douglas Harper
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Computing Dictionary

trigger definition

An action causing the automatic invocation of a procedure, for instance to preserve referential integrity. A triggers goes into effect when a user attempts to modify data with an insert, delete, or update command. A trigger can instruct the system to take any number of actions when a specified change is attempted. By preventing incorrect, unauthorized, or inconsistent changes to data, triggers help maintain the integrity of the database.
[Sybase SQL Server Release 10.0 Transact-SQL User's Guide].

The Free On-line Dictionary of Computing, © Denis Howe 2010
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American Heritage
Idioms & Phrases


In addition to the idiom beginning with trigger, also see quick on the draw (trigger).

The American Heritage® Dictionary of Idioms by Christine Ammer.
Copyright © 1997. Published by Houghton Mifflin.
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Example sentences
Certain risk factors, including a purine-rich diet, are also considered a
Professors, some of my students tell me, should not be trigger happy since they
  can make students extremely uncomfortable.
My theory has been that pheromones trigger my menstrual cycle to speed up.
Some small studies have suggested that eating less during a fever can trigger
  an immune response.
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