The quality of being appropriate, suitable, proper, or becoming; fitness.
From 'beseeming' plus '-ness' (abstract noun suffix), creating a noun that names the abstract quality of propriety and appropriateness.
This is the kind of word that feels quintessentially Victorian—turning a feeling of 'rightness' into a philosophical concept. English speakers loved bundling behavioral and aesthetic judgments into single abstract nouns.
Complete word intelligence in one call. Free tier — 50 lookups/day.