…one day with the plane that came blasting suddenly into sight out of the distant stillness and hurtled mercilessly along the shore line with a great growling, clattering roar over the bobbing raft on which blond, pale Kid Sampson, his naked sides scrawny even from so far away, leaped clownishly up to touch it at the exact moment some arbitrary gust of wind or minor miscalculation of McWatt’s senses dropped the speeding plane down just low enough for a propeller to slice him half away.
There are no more uses of "arbitrary" in the book.
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It was an arbitrary decision.
The speed limit seems to be arbitrary. I can’t imagine why it is so slow.