From sugar + baby.

sugar baby (plural sugar babies)

1. The younger recipient (usually female or a bottom) of interest from a sugar daddy or sugar mama, especially financial and romantic in an intergenerational relationship
   • 2008, Lisa Kleypas, Sugar Daddy, link ↗:

     Sugar daddies prefer to think of it as a special friendship in which they provide gifts and help to a deserving young woman. And sugar babies convince themselves that a nice boyfriend should want to help out with his girlfriend, and in return she would naturally want to show her appreciation by spending time with him.

2. A term of endearment; sweetheart.
   • 1926, John Steinbeck, The Saturday Evening Post, Volume 198, link ↗

     "Sugar baby, daddy not only shot you — he tried to kill you!" A look of sweet ecstasy came into the wounded lady's eyes.

• (kept) kept man, kept woman, concubine, mistress
• (sweetheart) sweetie, honey, dear, sugar

• (antonym(s) of “kept”): sugar daddy, sugar mama, sponsor
• (antonym(s) of “sweetheart”): fucker, ass, dick, bitch, meanie

• Spanish: mantenido