Sentence:  The Pope in sending Relicks to Princes, does as Wenches do by their "Wassels" at "New-years-tide", they present you with a Cup, and you must drink of a <WSD>slabby</WSD> stuff; but the meaning is, you must give them Moneys, ten times more than it is worth. 
Meaning of the word marked with <WSD> tags:  Of a liquid: thick; viscous.