As described in the Wiki entry,
"In 1936, the Harlem Hamfats recorded "The Weed Smoker's Dream". Band member (Joseph) McCoy later rewrote the song, refining the composition and lyrics. The new tune, titled "Why Don't You Do Right?", was recorded by Lil Green in 1941, with guitar by William "Big Bill" Broonzy. The recording was an early jazz and blues hit.This cover of the classic is by Amy Irving for the 1988 comic film Who Framed Roger Rabbit? and is, in my opinion, the definitive version, succeeding the Peggy Lee rendition that was considered one of her signature tunes and the "standard" for this song.
The song has its roots in blues music and originally dealt with a marijuana smoker reminiscing about lost financial opportunities. As it was rewritten, it takes on the perspective of the female partner, who chastises her man for his irresponsible ways and admonishes him to:
Why don't you do right, like some other men do?
Get out of here and get me some money too..."
If you're interested, the Lee cover is here. It's a fairly straight-ahead big-band swing piece, and lacks the smoky insinuation that makes the Irving cover so tasty.
So...why don't you do right?