Brewer's: Falling Sickness
Epilepsy, in which the patient falls suddenly to the ground.
?Brutus.?He[i.e. Csar] hath the falling-sickness.?
?No, Csar hath it not: but you, and I, And honest Casca, we have the falling sickness.?
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894