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Synonyms for Unnatural (same or very similar meaning)

WordNet sense 1 (distorted and unnatural in shape or size; abnormal and hideous):
grotesque, monstrous

WordNet sense 2 (displeasing to the senses):

WordNet sense 3 (lacking spontaneity; not natural):
forced, strained

WordNet sense 4 (artificially formal):

WordNet sense 5 (speaking or behaving in an artificial way to make an impression):

From the ODE community, based on WordNetadd/edit