Don't "cut someone down" but build them up. That's a good way to look at this verse. "Foul and abusive language" has the idea of degrading another person usually coming from someone whose character is deficient to start with. Also, consider the old adage, "sticks and stones may break my bones but words will never hurt me." That's simply not true! Just ask anyone who has been subjected to verbal abuse. Words hurt.
But, words can also "heal" and "build." That's what Paul says in contrast to unkind words. What you and I say can make such huge difference in the lives of another. That's certainly true of the Word of God, Micah 2:7b, "...Do not My words do good to him who walks uprightly?" NKJV. That's why David prayed, "Let the words of my mouth and the meditation of my heart be acceptable in Your sight, O Lord, my strength and my Redeemer." Psalm 19:14 NKJV.
Speak to lift up, never to put down.