A person who is clashed with; one who experiences a clash or confrontation with another person.
From clash (verb/noun) + -ee (suffix indicating the recipient of an action, as in employee, addressee). A back-formation creating an agent noun for the person acted upon.
The -ee suffix is incredibly productive in English for creating 'passive' agents—the clashee mirrors 'employee' structurally. But while we use 'employee' constantly, 'clashee' remains rare because it feels awkward to nominalize fleeting social conflicts.
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