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.
If a person has the devil's own luck, he or she has extremely good luck.
A: Yesterday I won a $10 million lottery jackpot and today I won a car in a draw.
B: You have the devil's own luck.