Use the adjective clandestine to describe something that is done in secret, like your clandestine attempts to steal your brother's Halloween candy.

Clandestine, an adjective imported from Latin, describes a secret, usually illegal activity. Often these things have to do with political and spy organizations. The CIA might run a clandestine operation to infiltrate terrorist organizations. Words with similar meanings include secret; covert, used especially for military matters; undercover, common in espionage; and surreptitious which emphasizes a person's efforts to keep something secret.

Definitions of clandestine
  1. adjective
    conducted with or marked by hidden aims or methods
    clandestine intelligence operations”
    synonyms: cloak-and-dagger, hole-and-corner, hugger-mugger, hush-hush, secret, surreptitious, undercover, underground
    secret or hidden; not openly practiced or engaged in or shown or avowed
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