undermine somebody's morale
weaken somebody's resolve

Idle hands do the devil's work, which in this case means undermining morale.

Is your company alert to the fact that the fear of physical violence at work can seriously undermine efficiency and morale?

Kielland was seen as undermining Christian authority and morale in general.

As the news spread through the U-boat fleet, it began to undermine morale.

Federal army generals were often corrupt and guilty of undermining morale with poor leadership.

This can undermine morale, establish bad attitudes, and often result in deviant or, even violent behavior.

Just a little something extra to undermine human morale.

And therewith the film would have begun to undermine morale.

Conversely, it can also serve to undermine morale and raise expectations too high for the company to meet.

"The colonel issued me a direct order to get my wife under control before she undermined morale."