This song is about a guy trying to buy his buddy a drink, which might seem like poor taste considering their lead singer, Bon Scott, drank himself to death
just months before the album was released. His replacement, Brian Johnson, stayed true to Scott's form by writing about topics consistent with AC/DC's ethos, and that includes drinking. Many bands would have released a heartfelt ballad to honor a fallen member, but that's not the AC/DC way - Bon would have wanted them to rock out, and that's exactly what they kept doing.
Ryan - Hesperia, CA