In Earth slang, the terms "ladies' man" or "lady killer" generally referred to a man who often enjoyed flirting with, or who was found attractive by women, sometimes with the intention of pursuing sex. The term "mack", which was popular during Earth's 20th century, was nearly as synonymous, with focus more on the latter part of the stated definition.
In 2268 Benjamin Sisko told Jadzia Dax that James T. Kirk was a ladies' man, leaving him with the impression that she found Kirk's eyes attractive. She actually was referring to Spock. (DS9: "Trials and Tribble-ations")
Watching events that happened in 2327, seeing a young Jean-Luc Picard strike out with Penny Muroc, Q remarked that he had no idea Picard was such a ladies' man. Picard replied that he was just a hormonal adolescent. (TNG: "Tapestry")
In 2368, Ira Graves described himself as having been a lady killer in his youth. Data misunderstood and thought he was condoning homicide. When pressed for an explanation, he said that it meant he was once as beautiful as he was smart, but when Data questioned this, admitted it was not really true. (TNG: "The Schizoid Man")
As of 2369, William T. Riker had the reputation of being a ladies' man. Calvin Hutchinson recognized Riker at first glance and informed the first officer that his reputation preceded him. (TNG: "Starship Mine")
In 2372, Gul Dukat described Shakaar Edon and Gul Marratt as both being ladies' men. The latter in particular had a penchant for chasing other officers' wives, including Dukat's wife. For this infraction, Dukat planned to demote him and assign him to the Cardassian embassy on Breen. (DS9: "Return to Grace")
In 2374, Neelix, recalling that Tom Paris used to be a ladies' man, asked the Human's advice about his plan to woo Talli. The newly involved Paris was depressed by Neelix's use of the past tense. (VOY: "Random Thoughts")