Bated breath

World Wide Words says:

Q: From Steve Gearhart: Where does the term baited breath come from, as in: ‘I am waiting with baited breath for your answer’?
A: The correct spelling is actually bated breath but it’s so common these days to see it written as baited breath that there’s every chance that it will soon become the usual form, to the disgust of conservative speakers and the confusion of dictionary writers.
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It’s easy to mock, but there’s a real problem here. Bated and baited sound the same and we no longer use bated (let alone the verb to bate), outside this one set phrase, which has become an idiom. Confusion is almost inevitable. Bated here is a contraction of abated through loss of the unstressed first vowel (a process called aphesis); it means “reduced, lessened, lowered in force”. So bated breath refers to a state in which you almost stop breathing as a result of some strong emotion, such as terror or awe.