The Collaborative International Dictionary of English v.0.48:

Falstaff \Falstaff\ prop. n.
   Sir John Falstaff, a celebrated character in Shakespeare's
   historical play " Henry IV." (1st and 2d parts), and also in
   " The Merry Wives of Windsor." He is a very fat, sensual, and
   witty old knight; a swindler, drunkard, and good-tempered
   liar; and something of a coward. Falstaff was originally
   called Sir John Oldcastle. The first actor of the part was
   John Heminge.
   [Century Dict. 1906]

   Note: Shakespeare found the name of John Oldcastle in the ...
         older play of "Henry V."; in the Chronicle he found a
         John Oldcastle, who was page to the Duke of Norfolk who
         plays a part in "Richard II."; and this, according to
         Shakespeare, his Falstaff (Oldcastle) had been in his
         youth. When the poet wrote his "Henry IV." he knew not
         who this Oldcastle was, whom he had rendered so
         distinct with the designation as Norfolk's page; he was
         a Lord Cobham [Sir John Oldcastle, known as the good
         lord Cobham], who had perished as a Lollard and
         Wickliffite in the persecution of the church under
         Henry V. The Protestants regarded him as a holy martyr,
         the Catholics as a heretic; the latter seized with
         eagerness this description of the fat poltroon, and
         gave it out as a portrait of Lord Cobham, who was
         indeed physically and mentally his contrast. The family
         complained of this misuse of a name dear to them, and
         Shakespeare declared in the epilogue to "Henry IV."
         that Cobham was in his sight also a martyr, and that
         "this was not the man." At the same time, he changed
         the name to Falstaff, but this was of little use; in
         spite of the express retraction, subsequent Catholic
         writers on church history still declared Falstaff to be
         a portrait of the heretic Cobham. But it is a strange
         circumstance that even now under the name of Falstaff
         another historical character is again sought for, just
         as if it were impossible for such a vigorous form not
         to be a being of reality. It was referred to John
         Fastolfe, whose cowardice is more stigmatised in "Henry
         VI." than history justifies; and this too met with
         public blame, although Shakespeare could have again
         asserted that he intended Fastolfe as little as Cobham.
         --Gervinus, Shakespeare Commentaries (tr. by K. E.
         Bunnett, [ed. 1880), p. 800.
         [Century Dict. 1906]

WordNet (r) 3.0 (2006):

Falstaff
    n 1: a dissolute character in Shakespeare's plays [syn:
         Falstaff, Sir John Falstaff]