AS RUBEN SAYS, IN SPAIN WE USE IT TO REFER TO AN ILLNESS, SICKNESS, DIZZINESS THAT CAME SUDDENLY WITHOUT EXPECTATIONS LIKE. BUT ALSO IT CAN BE USE TO EXPRESS HOW SOMETHING IMPRESSED YOU SO BADLY THAT YOU HAD A "JAMACUCO", A BIG SHOCK. I WOULD TRANSLATE IT FOR "SHOCK" OR THE EXPRESSION "IT NEARLY KILLED ME"

Cubans use it to means something that comes on when you least expect it. You hear: No trabajes tanto que te va a dar un jamacuco. I've also heard: Le dio un jamacuco epiléptico. -or- No toques ese tema con tu padre que se enfurese y le da un jamacuco. So it's a sudden fit, which could be anything from a tantrum to a fatal heart attack.

it's a slang word we use to say about a suddenly illness as a sickness, a dizziness or something more serious as a heart attack.