Please define "dirty old man"

"Dirty old man" is a post-Depression Era expression that was used by women of the socially elite during the 1930s to describe any man who was older than her daughter who would be interested in her baby girl, a girl who could be a 25 to 30 year old unwed young woman who still lived at home with her parents, but attended social functions with them looking for Mister Right.

The "dirty" part is in reference to rich older women looking down on less-than-rich younger men who would dare to attempt to worm his way into their social circles through his apparent connection with the daughter.