He was now decently clothed in a "sport shirt," open at the neck, sneakers, and duck trousers of a nebulous hue.
There are no more uses of "nebulous" in the book.
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... all of which fell into the nebulous category of we’re-building-up-God’s-house,
James McBride -- The Color of Water
…hunting on Sunday; war on the increasing tendency of young women to paint themselves and smoke in public; war on drinking whiskey—in this connection at least fifty children per summer went to the altar and swore they would not drink, smoke, or curse until they were twenty-one; war on something so nebulous Jean Louise never could figure out what it was, except there was nothing to swear concerning it; and war among the town’s ladies over who could set the best table for the evangelist.