What does it mean, “an idle mind is the devil’s workshop?”

Answer:

There is a set of English proverbs that are worded similarly:

• Idle hands are the devil's workshop
• Idle hands are the devil's tools ("Collections" 1808)
• Idle brains are the devil's workhouses (T. Fuller 1732)
• An idle brain is the devil's workshop (H. G. Bohn, "Hand-Book of Proverbs," 1855)
• If the Devil finds a Man idle, he'll set him at work (J. Kelly, "Scottish Proverbs," 1721)
• The devil finds work (or mischief) for idle hands to do

There are many variations, but they all follow the same theme: A person who doesn't have something particular to occupy himself with doing will be tempted to occupy himself with sin. The phrasing is not found in the Bible, but like many "chimney corner scriptures," it has its roots in a scriptural idea.

Idleness means you have no particular goal in mind, and, thus, you can be easily distracted. A warning is given about supporting young widows from the church's funds because it leaves the woman with little to do. "And besides they learn to be idle, wandering about from house to house, and not only idle but also gossips and busybodies, saying things which they ought not" (I Timothy 5:13). For this same cause, all Christians are commanded to work. "For even when we were with you, we commanded you this: If anyone will not work, neither shall he eat. For we hear that there are some who walk among you in a disorderly manner, not working at all, but are busybodies. Now those who are such we command and exhort through our Lord Jesus Christ that they work in quietness and eat their own bread" (II Thessalonians 3:10-12).