Tā kuīsì duōnián, zhōngyú zhǎodào le xiàshǒu de jīhuì.
After years of lying in wait, he finally found his chance to strike.
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Negative connotation — strongly suggests bad intent (spying, plotting, predatory waiting). Don't use it for innocent observation. Both characters carry the 'covert watching' sense: 窥 'peep' + 伺 'wait and watch'.
