demoralize

from The American Heritage® Dictionary of the English Language, 5th Edition.

• transitive verb To undermine the confidence or morale of; dishearten.
• transitive verb To put into disorder; confuse.
• transitive verb To debase the morals of; corrupt.

from The Century Dictionary.

• To corrupt or undermine the morals of; weaken or destroy the effect of moral principles on.
• To deprive of spirit or energy; dishearten; destroy the courage, confidence, or hope of; render incapable of brave or energetic effort: specifically used in relation to troops: as, the charge of our cavalry completely demoralized the enemy's left wing.
• To throw into confusion in general; bring into disorder; confuse mentally: as, he was badly demoralized by fright.
• Also spelled demoralise.

from the GNU version of the Collaborative International Dictionary of English.

• transitive verb To corrupt or undermine in morals; to destroy or lessen the effect of moral principles on; to render corrupt or untrustworthy in morals, in discipline, in courage, spirit, etc.; to weaken in spirit or efficiency.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• verb American To destroy morale; to dishearten.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• verb corrupt morally or by intemperance or sensuality
• verb lower someone's spirits; make downhearted
• verb confuse or put into disorder