Answer:
One thought:
"An idle mind is the devil's playground" is not found specifically in the Bible. It is an English proverb, originally rendered as "Satan finds some mischief still for idle hands to do". The principle involved, however IS found in the Bible.
We are told, in 1 Corinthians 15:58 " . .Therefore, my beloved brethren, be steadfast, immovable, always abounding in the work of the Lord, knowing that your toil is not in vain in the Lord", so we should always be BUSY doing what's right.(Ephesians 4:28) "Idle men" were often spoken of as insolent and lawless, and were used by God's enemies to form mobs to attack others (Judges 9:4)(Acts 17:5)(Proverbs 15:2), much as we see happening today. Those who are idle, having no purpose and nothing to do(mentally or physically),can get themselves into trouble. (1Timothy 5:13)"...and not merely idle, but also gossips and busybodies, talking about things not proper to mention. " NASB(2Thesolonians 3:11). These 'meddlers' are considered by God to be as bad as "...a murderer, a thief, and an evildoer"(1Peter 4:15) because of the division and chaos they can cause. Satan can certainly use someone who is bored, angry and has 'no where to go' mentally, and point him in HIS direction. (Proverbs 18:1) Philippians 4:8 tells us what to keep our minds on when it says:"...Finally, brothers and sisters, whatever is true, whatever is noble, whatever is right, whatever is pure, whatever is lovely, whatever is admirable-if anything is excellent or praiseworthy-think about such things. Whatever you have learned or received or heard from me, or seen in me-put it into practice. And the God of peace will be with you."(NIV)(Colossians 3:2)(Galatians 5:22)