behind (someone's) back

if you do something behind someone's back, you do it without them knowing, in a way which is unfair.

I don't want to talk about it behind his back.

She was accused of going behind her colleagues' backs to talk to management.

• the mother of all
• the mother of all something
• the mot juste
• the movers and shakers
• the moving spirit
• then again
• the name of the game
• then and there
• the nature of the beast
• the new kid on the block
• the next rung on the ladder
• the nightmare scenario
• the nitty-gritty
• the nouveau riche
• the nuts and bolts
• the odd man out
• the odd one out
• the Old Bill
• the old-boy network
• the old country
• the oldest profession
• the oldest profession in the world
• the oldest trick in the book
• the old guard
• the old school tie