It's pronounced 'ay jiss' not 'ay giss.' This is a real word, and the fact that there's still people that are in the gang after 2 years not pronouncing it right is a disgrace to everything that is milsim and an embarrassment to your own gang.
It is a real word. Its pronunciation is not open to interpretation. It is a specific Greek shield mentioned in the Illiad derived from Zeus' shield, that became a figure of speech for 'under protection' to the current most common usage, the A.B.M.D.S. (Aegis Ballistic Missile Defense System) that is used to stop incoming ballistic and potentially ICBM systems.
Now go about your lives pronouncing it correctly and pretending you always knew how to pronounce it correctly, and don't stop for a second to thank me for helping to fix you sounding like a moron. Thank you.