in Dryden's famous satire, is Thomas Shadwell, poet-laureate, whose immortality rests on the not very complimentary line, “Shadwell never deviates into sense.” (1640-1692.)
N.B. Flecknoe was an Irish Roman Catholic priest, doggerel sonneteer, and playwright. Shadwell, according to Dryden, was his double.
The rest to some slight meaning make pretence, But Shadwell never deviates into sense.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on MacFlecknoe from Fact Monster: