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"I saw the diamonds in the showcase, locked it, left the store, and armed the alarm."
This is an uncommon use of "armed" which can mean to prepare the fuse on a bomb, or something similar, so it is ready to explode.
Definition (verb 1.2): http://www.oxforddictionaries.com/us...lish/arm#arm-2
In this case it means to turn on the alarm so that it will ring (or whatever) when there is a burglary.
A more common phrase is "set the alarm".
On a passenger plane you might hear the captain tell the cabin crew to "arm the doors". This means to set the doors so that they make a power assisted opening in an emergency.
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