To someone is to undermine their confidence, spirit, or hope. It is less about physical defeat and more about winning the "inner war" by making an opponent or individual feel that their efforts are futile [1, 2]. Core Meaning
Constant changes in rules or expectations can make a person feel they can never win.
The person no longer sees a reason to keep trying.
Making someone feel like they are fighting alone often breaks their spirit faster than any physical challenge. Synonyms and Nuance