Brewer's: Falling Sickness
Epilepsy, in which the patient falls suddenly to the ground.
“Brutus.—He[i.e. Cæsar] hath the falling-sickness.”
“No, Cæsar 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