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spy on

B1 neutral inseparable transitive
In simple words

To secretly watch someone to find out what they are doing

Meanings

1 B1 neutral

To secretly watch a person or group in order to gather information about their activities

"The agency was accused of spying on its own citizens without legal authorisation."

"The NSA has been spying on millions of ordinary Americans."

— Edward Snowden, interview with The Guardian, June 2013
Grammar: inseparable
2 B1 informal

To watch someone you know in a secretive and intrusive way out of jealousy, suspicion, or nosiness

"She caught her younger brother spying on her through the keyhole."

Grammar: inseparable
Usage notes

Always involves secrecy and is almost always considered negative or intrusive. Used in contexts ranging from governments and agencies to jealous partners and curious children. Common in news, politics, and spy fiction.

Commonly used with

citizens rivals government neighbours employees suspect

Forms

Base
spy on
I/you/we/they
3rd person
spies on
he/she/it
Past simple
spied on
yesterday
Past participle
spied on
have + pp
-ing form
spying on
continuous

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Synonyms

surveil monitor snoop on watch tail shadow

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