A proverb meaning that even among close friends one should observe politeness and decorum; familiarity doesn't justify rudeness.
親しき仲にも礼儀あり、だから友だちの前でも最低限の節度は守ったほうがいい。
Even among close friends, there should be manners — so it's best to keep basic decorum even in front of friends.
祖母はよく『親しき仲にも礼儀あり』と言って、礼儀を大切にするよう教えた。
My grandmother often said '親しき仲にも礼儀あり' and taught me to value manners.