It is the intention to cover up one's evil through stubbornness when one's spiritual friends or others, in their desire to help one, raise the point of one's evil.What is slyness-concealment? It is to perpetuate a state of unresolvedness [kha na ma tho ba] because of its association with dullness and stubborness [gti mug] when one is urged towards something positive. Slyness-concealment has the function of preventing one from making a clean break with it and feeling relieved.-Abhidharmasamuccaya
By concealing one's evil and not admitting it, evil, however small, will grow bigger and bigger and will provide a cause for not allowing one to make a clean break with it and feel relieved. It will throw one into evil forms of existence later one.