This is from a text about covid-19 immunity.
https://www.cbc.ca/learning-english/2020-immunity-passports-level-2-transcript-1.5593750
'Personal trainer Chad Warren was knocked right off his feet in late March.
Now, he wants to know if he has immunity.'
Literally, if someone is "knocked off their feet", it means that they fall to the ground due to being pushed or hit. In this context, however, the expression is apparently used figuratively to mean that he became very ill (with the virus).
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Literally, if someone is "knocked off their feet", it means that they fall to the ground due to being pushed or hit. In this context, however, the expression is apparently used figuratively to mean that he became very ill (with the virus).