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Synonyms for Unimportant (same or very similar meaning)

WordNet sense 1 (of little substance or significance):

WordNet sense 2 (having little importance):

WordNet sense 3 (too small to make a significant difference):

WordNet sense 4 (not important or noteworthy):

WordNet sense 5 ((informal) small and of little importance):
little, petty

WordNet sense 6 ((often followed by `to') lacking importance; not mattering one way or the other):
immaterial, indifferent

WordNet sense 7 (lacking worth or importance):
inconsequential, inconsequent

From the ODE community, based on WordNetadd/edit