someone's back is turned

US /ˈsʌm.wʌnz bæk ɪz tɜrnd/
UK /ˈsʌm.wʌnz bæk ɪz tɜrnd/
"someone's back is turned" picture
1.

誰かが背を向けている間に, 誰かが注意を払っていない間に

when someone is not looking or paying attention

:
The children started playing pranks as soon as their mother's back was turned.
母親が背を向けた途端、子供たちはいたずらをし始めた。
He always tries to sneak a cookie when my back is turned.
私が背を向けていると、彼はいつもこっそりクッキーを取ろうとする。