Midst means the middle of.(preposition)
When you are in the middle of cleaning, this is an example of a situation where you are in the midst of cleaning.
Midst is defined as a middle point or middle part.(noun)
The point right in the middle of a city is an example of the midst of the city.
See midst in Webster's New World College Dictionary
Origin: ME middest, prob. merging of middes, gen. of mid (with unhistoric -t) + middest, superl. of mid, mid
See midst in American Heritage Dictionary 4
Origin: Middle English middes, middest
Origin: : alteration of Old English midde, middle; see medhyo- in Indo-European roots.
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