catch (someone) napping

Translations
застать (кого-то) врасплох
The sudden storm caught the hikers napping.
Внезапная буря застала туристов врасплох.
Definitions
To surprise someone who is not paying attention or is unprepared, often resulting in them being caught off guard or at a disadvantage.
Words
nap

verb
дремать
catch

verb
поймать