prick-song: Music sung from written or printed vocal music, as opposed to music sung from memory or by ear . . . Oxford English Dictionary.
Mercutio's satirical point is that Tybalt fights by the book, in the most fashionable manner. In Shakespeare's time there were many duelling manuals from which Tybalt could taken his lessons.
An illustration from the rapier fencing manual of Vincentio Saviolo