Very old English nouns may exhibit an irregular relationship between the singular and plural forms, as in foot and feet, mouse and mice. But most nouns construct the plural form in a simple, predictable way from the singular form. Rarely, though, one finds a word that is, or seems to be, a plural form, which would be judged to have been created in the regular way, except that there is scanty evidence of the presumptive singular form.
to be in cahoots
to be in the doldrums
to drain the dregs
to be down in the dumps
to have the heebie jeebies
the hustings
to merit kudos
the measles
the mumps
the munchies
to suffer from the runs
the screaming mimi's
the willies
the yaws