Less than zero, most commonly used to refer to temperature.(adjective)
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Use "subzero" in a sentence
"Look at all the folks who are in subzero temps right now and still adamantly maintain they need/love the change of seasons so would never move to Mexico."
"What'll be more impressive is to see him out in subzero weather shaking hands at the L stations."
"I dropped my first goose in subzero weather when the lower river was frozen over completely (a rare occurrence even in those days)."