A word there is of plural number, 
Foe to ease and tranquil slumber,
Any other word you take, 
And add an s will plural make. 
But if you add an s to this, 
So strange the metamorphosis; 
Plural is plural now no more, 
And sweet what bitter was before. 

What word is referred to in this riddle?

*** Originally appeared in a collection composed by William S. Walsh in 1892