MONICA: Joey, Stop Hitting On Her! It’s Her Wedding Day!
To “hit on someone” is to flirt with them, or seduce them.
It is not the same as to “hit someone” which implied some sort of physical violence. 😅
According to this in-depth report, the term goes back at least as far at 1954.
Hit on: (1940’s–1950’s) to make a request, especially for love-making.
Clarence Major, Dictionary of African American Slang (1970)