In The Blacklist, episode S03E05, about 27:30 in, a guy asks something like this.
Some say it's a daddy-daughter thing.
Others swear it's a may-september...
What does it mean? I haven't heard that reference before.
A May-September romance is a relationship in which one partner is significantly older than the other. May is in the spring and represents vitality and youth, perhaps a 20-30 year old person. September is the beginning of autumn and represents a person entering their later years, perhaps 40-60.
You can also use May-December, particularly for relationships with extreme age differences, perhaps a 20 year old marrying an 80 year old.
There is no implied gender relationship between the ages; it is used both for young woman + old man and young man + old woman.