Go behind (someone's) back verb phrase
Do something secretly when someone is not present
He went behind his wife's back and betrayed their marriage.
He is untrustworthy as he often goes behind his friends' back and gossips about them.
You should be open with your parents about it rather than go behind their back.
The verb "go" must be conjugated according to its tense.
To make love or have sex with someone
They just jumped each other’s bones after one date.