The wording of the definition makes it sound like an adjective, but it IS probably the plural of just another spelling of parbuncle, parbunkel, parbunkle, all variants of the noun parbuckle.
The promoter of the word has it coming from no less than Captain John Smith (1580-1631) in A Sea Grammar, With The Plaine Exposition of Smiths Accidence for young Sea-men, 1627. All but this spelling appear in the OED.
"A 17th century English word that means “coming together through the binding of two ropes,” according to a 1627 publication housed at the New York Public Library’s Rare Book Division, was, until this month, dead to the digital world—and to almost every living person." The Word The Internet Didn't Know