IPA: /dɪsəˈpɔɪntɪŋli/
KK: /dɪsəˈpɔɪntɪŋli/
In a way that causes disappointment or sadness.
The movie ended disappointingly, leaving many viewers unsatisfied.
Disappointingly → It is formed from "dis-" (meaning apart or away) and "appoint" (from Latin "appointare", meaning to set or place). The suffix "-ingly" indicates a manner of action. The word "disappointingly" means in a manner that causes disappointment or sets apart from expectations.
Think of being 'set apart' ('dis-') from what was 'expected' ('appoint') — that's why something is disappointing.