What is the historical origin of the "Last Supper" expression? This is not a question about the meaning (self-explanatory) of the expression, but on its origin. In particular, about who, where, and when. 

There seems to be nothing about this around. The New Testament does not mention these words. Wikipedia offers no work on its origin, and I find no online answer for this.
Doing a quick search of Patrology, I found the expression ultimam coenam in St Augustine of Hippo's tractatus 109 on Jn 17:20:


  
  Dominus Iesus iam sua propinquante passione, cum orasset pro discipulis suis, quos et Apostolos nominavit, cum quibus coenaverat ultimam coenam...
  
  The Lord Jesus, his Passion already approaching, after having prayed for his disciples, whom he had named Apostles, and with whom he ate the Last Supper...
  


So the term dates at latest to the beginning of the fifth Century, which is when St Augustine refers to it. He does use it offhandedly, though, which suggests the term was already in current use by then.