"For God's sake" is an exclamation used to show frustration, urgency, or emphasis. People say it when pleading, scolding, or reacting to something annoying or shocking. It's informal and can sound strong; softer versions include "for goodness' sake".
For God's sake hold your tongue, and let me love. John Donne
For God's sake (I never was more serious), don't make me ridiculous any more by terming me gentle-hearted in print. Charles Lamb
Everything's funny for God's sake. Everything. Gloria Estefan
I'm a comedian, for God's sake. Viewers shouldn't trust me. And you know what? They're hip enough to know they shouldn't trust me. I'm just doing stand-up comedy. Dennis Miller
No woman marries an old man for God's sake. German Proverb
If the Devil were dead, folk would do little for God's sake. Scottish Proverb