To bluejack someone is to send a short text to a phone near you through Bluetooth, without saying who you are. The other person just sees a surprise message. People did it as a joke, mostly in the 2000s, on trains and in cafés, when phones were easy to find over Bluetooth. The word is also used for the practice itself. It is a prank, not a break-in: you are not opening their files or stealing anything. Informal, a bit techy, and rather old-fashioned now; newer phones are less open to strangers. You will meet it in old news more than in today's chat. The person who does it is a bluejacker; the act is also called bluejacking.

藍芽惡作劇傳訊（台灣說「藍芽」，中國大陸寫「藍牙」；不是駭客入侵）