The song is about a girl who doesn't appreciate what her man gives her. The "dirty lowdown" is the honest truth - what Scaggs is encouraging this poor sap to face.

 

TRIVIA:

The word "Lowdown" was popular slang meaning a summary of what's going on for real. The first Hot 100 entry with the term in the title came in 1969 with the instrumental "Lowdown Popcorn" by James Brown (#41, 1969). Next came Chicago's song "Lowdown" (#35, 1971).