SPY - [verb/noun] to watch somebody or something in a secret way; a person employed to find out some secret information
Examples of use
The old lady is always spying on all her neighbours.
Nobody expected that friendly Peter could be a spy who just wanted to know our plan of action to use it against us.
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English speaking practise
Remember that if you don't use that word in English conversations or writing you'll forget it in a short time. So the best thing you can do to learn this word is just trying to use it in speaking or writing. Remember also that practise is the key to fluent english speaking.
You can always find a partner, who would also like to improve his/her