The word ma'am is a contraction of madam which is typically used as a polite form of address for a female.
There are no categorical antonyms for this word.
However, one could loosely use insulting forms of address for a female as antonyms, e.g., bitch, harpy, shrew, etc.
Opposite of plural for a respectful term of address to any male, especially if his name or proper title is unknown
“So, if I could put it this way, ma'am, it is not only the quantity of your reign we are celebrating today, it is the quality.”