Lyric discussion by floydzkrieg 

Cover art for Today's Lesson lyrics by Nick Cave and the Bad Seeds

I think the song is generally about a young girl and her much older drug dealer, who is referred to in the lyrics as "Mr. Sandman".

The first line where it says "Little Janie, she wakes up from a dream..." could be talking about her coming done from a drug high, which would make "Mr. Sandman" her dealer as in conventional meaning Mr. Sandman is the one who sends us of to sleep and gives us dreams. Also note where it says "There oughta be a law against me going down on the street". Why should such a thing be true unless he was engaged in illegal activity?

Janie may also be in some sort of trouble with her Sandman as is evident in the lyrics "Mr Sandman runs around the corner/Trying to head her off at the pass/He sticks his head over the fence and yells/Something way to fast...". Perhaps she owes him money?

She's a cautious girl, though. The "jawbone" done the waistband of her jeans is undoubtedly a gun as they are vaguely shaped like the jawbones of certain four-legged animals.

There's also a lot of sexual desire in the Sandman for Janie as can be seen in virtually the entirety of the last verse.

I dunno, there's a lot to read into hear. A LOT.

You'll have to excuse all the spelling errors there. Long day lol.