When did the term "New Testament" first come into usage? While reading William Bridge's, "The Wounded Conscience Cured," (1642) I came across Bridge's usage of the term "New Testament" :


  "And in the New Testament, Romans 13.1..." 
  
  -William Bridge, "The  Wounded Conscience Cured," (5)


While obviously not the earliest usage of the term "New Testament," this did get me wondering about how early on the term first came into usage. 



David Trobisch, a prominient scholar on the formation of the Christian Bible, records in his book what he deems to be the earliest usage of the term in an anonymous late second-century treatise against the heresy of Montanism :  


  "At approximately the same time, (late second century) the
  previously mentioned anonymous  treatise against the Montanist
  movement seems to use the term New Testament in reference to a written
  source. The author explains that  he was initially very reluctant to
  write at all. He did not want to  create 'the impression that he was
  adding anything to the word of the  gospel of the New Testament;
  since no one, who decided to live  according to the gospel, can add to
  it or take away from it.'" 
  
  -David Trobisch, "The First Edition of the New Testament", (44)




With regards to answers, "New Testament" does not have to be used in reference to a modern day canon; however, "New Testament" should inherently be used to describe some sort of canon from antiquity. 

Therefore,

I am curious if there are any other earlier usages of the term "New Testament" or is David Trobisch's example truly the earliest, extant usage of the term? 

Just to clarify, I am not inquiring of the earliest English usage of the term, but earliest in any language, so a usage resembling (but not exclusive to): Novum Testamentum, Καινή Διαθήκη, ܕܝܲܐܬܝܼܩܝܼ ܚܵܕܬܵܐ.
It seems that the first written usage of the term "New Testament" comes from the writings of Melito of Sardis (died c.180) and followed by Tertullian (c. 155 - c. 240). The term "New Testament" was coined in the second century.


  Etymology
  
  The term New Testament is a translation from the Latin Novum Testamentum first coined by the second century Christian writer Tertullian. It is related to the concept expressed by the prophet Jeremiah (31:33), that translates into English as new covenant:
  
  'The time is coming," declares the Lord, when I will make a new covenant with the house of Israel and with the house of Judah… '
  
  This concept of the new covenant is also discussed in the eighth chapter of the Letter to the Hebrews, in which the "old covenant" is portrayed as inferior and even defective (Hebrews 8:7). Indeed, many Christians considered the "old" covenant with the Jews to be obsolete. In Tertullian's day, some even considered the God of the Hebrew Bible to be a very different being than the Heavenly Father of Jesus. Tertullian took the orthodox position, that the God of the Jews and the God of the Christians are one and the same. He therefore wrote:
  
  "All Scripture is divided into two Testaments. That which preceded the advent and passion of Christ—-that is, the law and the prophets—-is called the Old; but those things which were written after His resurrection are named the New Testament. The Jews make use of the Old, we of the New: but yet they are not discordant… (Against Marcion)."
  
  While Christians have thus come to refer to the Hebrew Scriptures as the Old Testament, Jews prefer the term Hebrew Bible, or Tanakh, the latter word being an acronym for its three basic component parts: the Torah (Book of Moses), Nevi'im (Prophets), and Ketuvim (Writings). - New Testament (New World Encyclopedia)


The Catholic Encyclopedia has this to say the New Testament:


  Name
  
  Testament come from testamentum, the word by which the Latin ecclesiastical writers translated the Greek diatheke. With the profane authors this latter term means always, one passage of Aristophanes perhaps excepted, the legal disposition a man makes of his goods for after his death. However, at an early date, the Alexandrian translators of the Scripture, known as the Septuagint, employed the word as the equivalent of the Hebrew berith, which means a pact, an alliance, more especially the alliance of Yahweh with Israel. In St. Paul (1 Corinthians 11:25) Jesus Christ uses the words "new testament" as meaning the alliance established by Himself between God and the world, and this is called "new" as opposed to that of which Moses was the mediator. Later on, the name of testament was given to the collection of sacred texts containing the history and the doctrine of the two alliances; here again and for the same reason we meet the distinction between the Old and New Testaments. In this meaning the expression Old Testament (he palaia diatheke) is found for the first time in Melito of Sardis, towards the year 170. There are reasons for thinking that at this date the corresponding word "testamentum" was already in use amongst the Latins. In any case it was common in the time of Tertullian. - The New Testament