Synonyms for Unconventional (same or very similar meaning)
WordNet sense 1 (not conforming to legality, moral law, or social convention):
improper, unlawful
WordNet sense 2 (contrary to rule or accepted order or general practice):
irregular
WordNet sense 3 (conspicuously or grossly unconventional or unusual):
bizarre, eccentric
WordNet sense 4 (independent in behavior or thought):
unorthodox
From the ODE community, based on WordNet
add/edit