Initialism of Gas Injection System.
"The old slang dictionary listed gis as an alternative spelling of jizz, noting its use in mid-20th-century American speech."
plural of GI
Alternative spelling of jizz
In plain English: It's another word for bodily fluid released during sexual activity.
"The dog shook with excitement, spraying gis everywhere."
Third-person singular simple present indicative form of GI
"The gis in the military team are checking their equipment before dawn."
In plain English: To gis is to throw up.
"He gis on the couch after a long day at work."
Alternative form of gi's.
"The soldiers referred to their uniforms as gis in informal conversation."
The word gis comes from the military acronym GIS, which stands for general impression and shape. It entered English as a shorthand reference to this concept used by soldiers on the ground.