What is "sir john oldcastle"

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.

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.

Sir John Oldcastle is an Elizabethan play about John Oldcastle, a controversial 14th-15th century rebel and Lollard who was seen by some of Shakespeare's contemporaries as a proto- Protestant martyr.