Lady Luck was the personification of good fortune, described as being "with" someone who was lucky.
In 1893, the bellboy at the Hotel Brian asked a losing Mr. Lane if Lady Luck wasn't with him that night. (TNG: "Time's Arrow")
In 2366, a decidedly unlucky Geordi La Forge agreed to increase USS Enterprise-D's engine efficiency instead of continuing to play poker with his fellow crewmen, saying that Lady Luck had left him long ago. (TNG: "Allegiance")