A plurale tantum (Latin for "plural only"; plural form: pluralia tantum) is a noun that appears only in the plural form and does not have a singular variant for referring to a single object. These are used in English for objects that function as pairs or sets (glasses, pants, scissors, clothes, electronics, bagpipes, genitals).
词源
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plurale tantum:
From Latin plurale tantum ?(“plural as such, plural only”).