zločesto

1. Croat. (of behaviour) naughtily; wickedly; mischievously
   Nasmejao se zločesto i namignuo mi. — He grinned mischievously and winked at me.
   Deca su se prema njemu ponašala zločesto. — The children behaved wickedly towards him.

More characteristic of Croatian usage, but occasionally met in Serbian too. In Serbia the usual words are “nestašno” or “nevaljalo” (naughtily, mischievously), or “zlobno”, “pakosno” (wickedly, maliciously).