IPA: /ˌsʌb.rɛpˈtɪʃ.əs/
KK: /sʌbˈrɛp.tɪ.ʃəs/
Describing something that is done secretly or in a hidden way, often to avoid notice or attention.
The subreptitious actions of the spy went unnoticed by everyone around him.
Subreptitious is formed from "sub-" (meaning under) and "reptitius" (from "reptare", meaning to creep or crawl). The word describes something that is done in a sneaky or stealthy manner, as if creeping in under the radar.
Think of something happening 'under' ('sub-') the surface, like a sneaky action that 'creeps' ('reptare') in without being noticed.