A very effective way of doing most drugs. Basically, you stick it up your butt.
There's a stigma against it because a lot of people are scared of their own rectums. Ultimately, it's a matter of personal preference; if you're adventurous, try it- it works great. If not, there's plenty of other methods that don't make you feel violated.
I plugged Adderall just before I wrote this definition. And IMO, plugging turns a good high into a GREAT high.
You can plug pretty much anything except weed and DXM- but always Google it first, just in case.