1. attack or criticize somebody suddenly or unexpectedly
turn on somebody for no reason
The phrasal verb 'turn on' can mean to physically or verbally attack someone. If you 'turn on somebody' you treat them badly or aggressively usually suddenly or after being nice to them previously. (note: when you say 'to turn on somebody meaning to become aggressive towards them the stress in on the word 'turn' if you 'turn on somebody' and the stress is on the word 'on' it can have a very different meaning; 'to sexually excite someone', so you need to be careful with this expression.) To 'turn on' can also mean 'to have as the main or central point of discussion or interest', such as 'the conversation turned on to the environment'. You can 'turn on' something mechanical in order to start it or to get it to work, such as 'turn on the TV'. As a noun, 'a turn on' is something which is sexually exciting.