Macbeth Soliloquy Glossary: To be thus is nothing (3.1.47-71) My Genius is rebuk'd (58)
In Roman mythology, every man had a "genius" or guiding spirit. It was believed that one's genius could grant his worshippers special intellectual powers. Thus we have the modern definition of the word as "a person with exceptional intellectual or creative powers." Women had an equivalent spirit, known as a "juno."
Mabillard, Amanda. Macbeth Soliloquy Glossary. Shakespeare Online. 20 Aug. 2000. (date when you accessed the information) < http://shakespeare-online.com/plays/macbeth/soliloquies/genius.html >.