To discomfit someone is to make them lose their composure; to make them uneasy or embarrassed. You might notice an odd similarity to someone experiencing discomfort: This is not a coincidence. It is considered likely that the current meaning of the word resulted from people mixing it up with "discomfort". The original meaning (which is still correct, if rare) has to do with thwarting the plans of one's opponents. The meaning "to frustrate" falls somewhere in between the other meanings.
Interestingly, both comfit and discomfit come from the Latin word "conficere" (to prepare), but otherwise the words have little to do with each other: Comfit is a food consisting of spices or dried fruits or nuts coated with sugar.
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