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1. Something very important that one has been working or striving to achieve or win. Used after "the." OK, boys, this is the big one—the regional championships. Today we put our months of hard work to the test. This deal is the big one. Our company's entire future depends on us securing it.
2. Something extremely catastrophic or devastating. Used after "the." Analysts are warning that the destruction caused by the recent earthquakes would pale in comparison to the big one that could be coming down the line. It was only a minor heart attack, thank God, but the doctors say he might not survive if he's ever struck by the big one.
3. slang A unit of one thousand dollars. Used in plural constructions. The landscaper said it would cost five big ones to replace the sprinkler system. I spent 30 big ones on this damned car, and it's given me nothing but trouble since the day I bought it.
the big one
n. a serious heart attack, possibly fatal, especially if anticipated or dreaded. He survived the big one but kept on smoking.