The quality of being well-suited, appropriate, or bringing about happiness; the state of being felicitous.
From felicitous plus the noun-forming suffix -ness, which creates an abstract noun from an adjective. This is a standard English word-formation pattern.
The felicitousness of a phrase is what makes it memorable—it's when the form and meaning align so perfectly that the words seem inevitable, like you couldn't have said it any other way.
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