have half an ear on (something)

to listen to something without giving it all your attention.

I had half an ear on the radio as he was talking to me.

• be as quiet as a mouse
• be as regular as clockwork
• be as rich as they come
• be as right as rain
• be as safe as houses
• be as scarce as hen's teeth
• be as sharp as a tack
• be as sick as a dog
• be as sick as a parrot
• be as silent as the grave
• be as slippery as an eel
• be as snug as a bug in a rug
• be as sober as a judge
• be as solid as a rock
• be as sound as a bell
• be as sound as a dollar
• be as stiff as a board
• be as stiff as a ramrod
• be as straight as a die
• be as straight as a ramrod
• be as strong as an ox
• be as stubborn as a mule
• be a star in the making
• be as thick as thieves
• be as thick as two short planks