Green’s Dictionary of Slang

1. a black woman who is a regular church-goer and religious believer; cit. 1853 may have referred to Mary Low, daughter of the British publisher.

[1853

[bk title] A Peep Into Uncle Tom’s Cabin by ‘Aunt Mary’].

2. a downtrodden, subservient black woman.

1974

R. Carter Sixteenth Round 13: There were no such people in our young lives as Mister Charlie and the Devil — meaning white people — or Aunt Mary and Boy — meaning our mothers and fathers .