the quality of being becoming; suitability, grace, or attractiveness.
From 'becoming' plus the '-ness' suffix, creating an abstract noun. This represents the 17th-18th century habit of nominalizing adjectives to discuss qualities as philosophical concepts.
English lets you add '-ness' to almost any adjective to make it a noun representing that quality—'becomingness' is the abstract essence of being 'becoming,' a concept beloved by Victorian writers.
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