secretive
adj.keeping your thoughts, feelings, or actions hidden from other people. You use this to describe someone who does not like to share information about what they are doing.
adj.inclined to conceal one's activities, thoughts, or intentions from others. Often used predicatively after linking verbs like 'remain' or 'become'.
“He is very secretive about his new project.”
“The tech company is famously secretive about its upcoming products to prevent competitors from copying their designs.”
“While the administration promised total transparency, their secretive handling of the budget negotiations suggested a deep-seated reluctance to face public scrutiny.”
Often followed by the preposition 'about' when specifying the hidden information.
“A secretive WhatsApp group was pinging with notifications throughout Wednesday, letting its members know where in London the latest “drop” would take place.”
From Middle English secretife, equivalent to secret + -ive.
From secrete + -ive.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.