surreptitious (adj.)

mid-15c., from Latin surrepticius "stolen, furtive, clandestine," from surreptus, past participle of surripere "seize secretly, take away, steal, plagiarize," from assimilated form of sub "from under" (hence, "secretly;" see sub-) + rapere "to snatch" (see rapid). Related: Surreptitiously.