Ephesians 4:29
“Let no corrupt communication proceed out of your mouth, but that which is good to the use of edifying, that it may minister grace unto the hearers.”
Paul isn't just asking us to avoid crude words; he's asking for something more demanding: that every word have a purpose of building up. It's a higher standard than simply "don't be rude."
Let's not overcomplicate this: not insulting isn't enough. The goal is that what you say leaves the other person, present or absent, a little better than you found them.
Does the way I talk about others when they're not present build up or tear down?
Before saying something about someone who's absent today, ask yourself if that sentence would build something up if they were listening.
Lord, guard my mouth even when I speak about others who aren't in front of me. Let my words give grace, not harm.
Recommended reading: James 3:9-10