secret weapon

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Also see: weapon

Concise Oxford English Dictionary © 2008 Oxford University Press:
secret/ˈsiːkrət/

▶adjective not known or seen or not meant to be known or seen by others.

■ fond of having or keeping secrets; secretive.

▶noun

• 1 something kept or meant to be kept secret.

  ■ something not properly understood; a mystery: the secrets of the universe.

• 2 a way of achieving something, especially one that is not commonly known: the secret of a happy marriage is compromise.

• 3 a prayer said quietly by the priest after the offertory in a Roman Catholic Mass.

– derivatives
secrecy noun,
secretly adverb.

– origin ME: from OFr., from L. secretus ‘separate, set apart’, from secernere, from se- ‘apart’ + cernere ‘sift’.