When I was a child, I learned that the name Philadelphia derives from the Greek words philos (loving) and adelphos (brother) and that William Penn chose the name because he wanted to establish “a city of brotherly love.”
I’ve recently learned that the word philander derives from the Greek words philos (loving) and andro (man/male) and that in ancient Greek it meant “a lover of men.”
According to a note in the OED, the word in Hellenistic Greek could also be applied to a woman who loved the man who was her husband.
Philander was appropriated by 17th century poets as a stock name for a lover and came to be used for any man known for his popularity with women:
You know I was always a Philander among the ladies.” (OED citation, 1785)
In modern use, philander has become a verb that has in turn produced the agent noun philanderer.
philander (verb): to engage in casual sexual encounters. Used chiefly of men.
philanderer (noun): a man who philanders.
Both the OED and Merriam-Webster give flirt as a synonym for philanderer, but philanderer is more negative and gender-specific than flirt. Either a man or a woman may be a flirt, but a philanderer is a man.
Another difference is that flirting applies to a playful sort of sexual innuendo that does not assume sexual activity, while philandering connotes promiscuity and lack of marital or romantic integrity. Here are examples of recent use of philandering and philanderer:
Power and philandering seem to go hand in hand.
Actor Jon Hamm has played the philandering ad executive Don Draper on the American Movie Classics show Mad Men.
Unlike many presidents, Nixon had never been suspected of being a philanderer.
[In] 2007, Vitter was exposed as a paying philanderer.
Here are some additional words that are or have been used to describe men of loose character: