Synonyms for Screwball (same or very similar meaning)
WordNet sense 1 (a whimsically eccentric person):
crackpot, crank, fruitcake, nut, nutcaseWordNet sense 2 (foolish; totally unsound):
crazy, half-baked, softheadedWordNet sense 3 (not practical; not workable or not given to practical matters):
impracticalFrom the ODE community, based on WordNetadd/edit