subreptitious
/ˌsʌbrɛpˈtɪʃəs/
adjective
- Done or obtained by secret or dishonest means; stealthy or underhanded.
- They exchanged subreptitious glances while the teacher wasn't looking.
- His subreptitious behavior during the exam got him expelled.
- The spy made a subreptitious entry into the building after dark.