This was written by Jethro Tull frontman Ian Anderson, who has described the character Mary a "child prostitute." She lives a wretched existence offering her services to the dregs of humanity. The character "Aqualung" has a cameo in this song, presumably as one of Mary's cutomers. The "jack knife barber" is a man who performs abortions.
Who would be a poor man
A beggar man, a thief
If he had a rich man in his hand?
And who would steal the candy