armed the alarm

[unpakwon]

The meaning of "armed" in the following is confusing.

"I saw the diamonds in the showcase, locked it, left the store, and armed the alarm" leaves out the fact that the person put the diamonds in his duffle bag before he locked the showcase and left the store.

Thank you.

 

[JMurray]

not a teacher

"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/definition/american_english/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.

 

[Tdol]

Arm/disarm is sometimes used in BrE for alarms, though saying alarm the alarm sounds odd because of the repetition.