A spike, needle, the sharp part of an injection. Usually this slang is used by people on needle drugs to describe their experience with the needle without saying the word "Needle". Their friends and family may use it to try to reach out to them. Also many kids use this slang wether they've had the experience or not to seem cool and like they know what they're talking about. This was also a slang back in the day for needles.
"Heroin, is my life and it's my wife because a MAINER in my vein leads to center in my head, then I'm better off and dead!"
"DJ, the next time I see that MAINER touch your arm I'm leaving!"
"Man we have to try it just once, I just got this from him and I think we should shoot up with his MAINER, just tonight"
"I think Mrs.Altman has done the MAINER and she's lost it."