Origin
This proverb was first recorded in 1855.
Explanation
The human mind can never be inactive. When a man is idle, he is not engaged in any useful activity. His mind is then open to the influence of evil thoughts. The devil enters his mind and fills his mind with evil ideas. Thus his mind becomes the devil’s workshop. The only way to protect one’s mind from evil thoughts is to be engaged in good deeds. When a man is engaged in good deeds, his mind is filled with good thoughts. He has no time to think of evil. The people who have nothing worthwhile to think about will usually think of something bad to do. A person who sits and lazes around without doing any fruitful work is called an idle man. Such a man is not only wasting his energies but is also dangerous for the society. He gets dangerous ideas which are harmful not only to him but also to the whole society and so his brain is compared to the workshop of a devil.
Example
We need to figure out something constructive for Tom to do in the afternoon after school. An idle brain is the devil’s workshop.