To deprive of courage, confidence, or hope; to discourage or dishearten someone.
From Middle English dis- (reversal/negation prefix) + spirit (from Latin spiritus 'breath, life force'). The meaning developed from removing someone's vital spiritual force to the emotional sense of discouragement.
Notice how 'spirit' originally meant your breath or life-force, and 'disspirit' means to remove your motivation—our ancestors literally thought courage and energy were physical things that could be taken away!
Complete word intelligence in one call. Free tier — 50 lookups/day.