Today I Heard My First . . . . .
ChatterBank1 min ago
No best answer has yet been selected by shyted. Once a best answer has been selected, it will be shown here.
For more on marking an answer as the "Best Answer", please visit our FAQ.'Royal', as an adjective, always suggested something special or larger than life, as it were. Hence, a 'pair royal' was a very special twosome...ie a threesome!
In answer to Moog's point, 'rial' is a very old - 14th century - version of 'royal', so 'prial' makes perfect sense as a shortening of 'p(air)rial'.