Idioms are phrases which cannot be understood simply by looking at the meaning of the individual words in the phrase.
have (something) up one's sleeve
to have something kept secretly ready for a good time to use it
I am not too worried about the meeting as I have something up my sleeve if there are any problems.
List of common idioms, their meanings and examples of their usage in sentences
An idiom is a group of words, or in other words, a phrase that has a meaning other than its literal meaning of the words in it.