When we were asked to explain the story behind the verb pass out, we originally thought we抎 take a pass. After all, what could there be to say? But as so frequently happens, a little digging yielded some interesting information.

For starters, when pass out first passed into the lexicon around the turn of the twentieth century, it meant not to lose consciousness but to pass from physical life; expire; die. These days, such a sense is more commonly conveyed by pass, pass over, or pass on than by pass out.

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Then there抯 the transitive sense of pass out: to reject (a deal in bridge) as unplayable because everyone has passed on the first round of bidding.

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Pass has a dozen or so meanings, ranging from the obsolete to make a pass in fencing to the au courant circulate (as when bank notes pass so long as they are not refused) to the commonplace happen; occur (as when something comes to pass). (来源：英语问答中心 http://ask.englishcn.com)

And so it came to pass that pass out picked up its move into the realm of unconciousness sense by pairing the pass sense meaning to go from one quality, state, condition, or form into another with the sense of out associated with being in or into an insensible or unconscious state.