A combination of two different words, GENius and manIAC.
A brilliant individual who is completely and utterly out-of-control, possibly even insane.
If you encounter a geniac, do not attempt to emulate them, for it can only result in disaster.
If two geniacs come into contact, the results can vary from one extreme, spectacular mutual destruction, to the other extreme, extraordinary creative power.
Evariste Galois was definitely a geniac.
By age 20, he had failed out of school, been in prison, invented a new branch of algebra, and solved an unsolved mathematical problem.
At age 21, he was dead from a gunshot wound in a duel.