The quality or state of being disappointing; the degree to which something fails to meet expectations.
From 'disappoint' (from Old French 'desappointer') + '-ing' (present participle) + '-ness' (abstract noun suffix). The prefix 'dis-' means to reverse or negate, so 'disappoint' originally meant to remove from an appointed position, evolving to mean failing to fulfill expectations.
This word is so specific that it barely exists in real English—most people just say 'disappointment'! It shows how English lets us add '-ingness' to almost any adjective, creating these rare technical terms that linguists call 'productivity'—the ability to generate infinite new words from existing building blocks.
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