clandestine (adj.) Look up clandestine at Dictionary.com
1560s, from Latin clandestinus "secret, hidden," from clam "secretly," from adverbial derivative of base of celare "to hide" (see cell), perhaps on model of intestinus "internal." Related: Clandestinely. As a noun form, there is awkward clandestinity (clandestineness apparently being a dictionary word).