to not be listening to what someone is saying because you are thinking about something else.

'Jim, did you hear what I just said?' 'Sorry, I was miles away.'

• two-time
• two-timer
• two-time someone
• two-timing
• two-way street
• Uncle Sam
• under a cloud
• under a microscope
• under arrest
• under attack
• under belt
• under breath
• under certain circumstances
• under false pretences
• under fire
• under heel
• under lock and key
• under no circumstances
• under nose
• under own steam
• under pain of death
• under shadow
• under someone's heel
• under someone's nose
• under someone's thumb