Correct spelling, explanation: the word proper comes from Old French propre and from Latin proprius (meaning one’s own, special). As you can see, nothing in the etymology of this word indicates the necessity to put a double -p consonant in the spelling. Therefore, we should not write it as propper. If you are not sure what the correct form sounds like, it is worth checking the history of the word. Proper is not only an adjective but also an adverb.

Definition of proper:
1. adjective, real, satisfactory, suitable
You owe him a proper apology.
We should have a proper discussion before moving out.
2. adverb, complete, used for emphasizing what you’re saying
You have to do your homework properly because your future depends on it.
I was properly annoyed with her.

Collocations with proper:
Some most commonly used collocations include: perfectly proper and a proper way.
It’s perfectly proper to call the police when you see someone committing a crime.
Staying home is the proper way to avoid sickness.

Incorrect spelling, explanation: the word propper is a mistake – there’s no reason to spell it like that. If you look at its etymology, you’ll clearly see that the only correct version of this adjective/adverb is proper. It originates from Old French and Latin. Some people may get confused because it looks similar to pepper. Always remember to carefully check your spelling.