15e, f. Sometimes considered objectionable in standard usage, the idiom wait on meaning “to wait for, to await (a person)” is largely confined to speech or written representations of speech. It is most common in the Midland and Southern United States: Let's not wait on Rachel, she's always late. Wait on or upon (an event) does not have a regional pattern and occurs in a wide variety of contexts: We will wait on (or upon ) his answer and make our decision then. The completion of the merger waits upon news of a drop in interest rates.

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