Head on slugs

I'm just guessing here. I've never heard "don't do slugs" before. To me, this phrase is reminiscent of "kids, don't do drugs". Since " a slug" is a "a bullet", I'd be inclined to interpret the phrase as "don't use guns" and the picture possibly shows a helmet with a bullet hole in it.

However, "slug" can also mean "a shot of liquor" so perhaps the phrase means "stay off the booze" and t

I'm just guessing, too. As Ivan says, it's a play on 'Don't do drugs', but since that is the case, I'd go with the simple 'slug' = 'hit' or 'strike': the helmet has been damaged by being struck with a baseball bat or club of some kind.

Thank you, Ivan, Mr Micawber. I understand it now.

Regards
Jackson