"A man being polite to a woman," flexibly given an undefined negative conotation by the speaker if he is annoyed by the fact of the socialization but couldn't find any specific wrong to quickly point out.
A "fudge word," used to describe the action or inaction of a man who is as a man deemed to be the responsble party, to mean an inappropriately close communication or socialization between a man and a woman.
"Socializing," which is a positive word can be flexibly described neutrally and romantically as "flirting," or negatively as "hitting on."
Can also mean "hitting," thus further demonstrating the flexibile nature of its meaning.
"He was hitting on her." Means the speaker doesn't like that he was talking with her and wants to imply he was flirting with her against her wishes or in an inappropriately close way given the distance of their relationship or lack thereof or given the situation or setting in which the conversation arose (all more implied than specified or actual).
"He was hitting on his wife." Means "he was beating his wife."