plurale tantum
Definition - A word or phrase that is only used in its plural form.

Example -
The words willies, suds, and scissors.

Etymology -
In Latin the phrase means "plural only" or "plural as such."

Oxford English Dictionary -
Its first citation is from 1930:
"The so-called ‘pluralia tantum’ (e.g. breeches, dregs)."
(T. Sasaki Lang. R. Bridges' Poetry ii. iii. 57 )

Please comment