blue ‧ jack ‧ ing /ˈbluːdʒækɪŋ/ BrE AmE noun [uncountable]
the practice of using a ↑ mobile phone with ↑ Bluetooth technology to send a ↑ text message to another person near you, without letting them know who has sent the message. This is usually done as a joke to surprise the person who receives the message.
—bluejack verb [intransitive and transitive]
—bluejacker noun [countable]