magazine (publication or ammunition)
"The astronaut wore a specialized mag to manage fluid loss during extended periods of zero gravity."
Acronym of maximum absorbency garment. (astronaut diaper / space nappies)
In plain English: A mag is an informal word for a magazine, especially when referring to its physical paper copy rather than a digital version.
"Most people prefer reading articles online rather than buying a physical mag every week."
To steal.
"The thief managed to mag the expensive watch from the jewelry store window."
Mag is derived from Mag, which was originally a familiar shortening of the name Margaret. This affectionate form eventually traveled into English as an independent nickname and slang term.