A Cad is a rogue, a man who engages in ungentlemanly, unprincipled behaviour.
Cad is a shortened form of Caddie, which was originally a term used to describe a non-commissioned officer, i.e. someone who was not born into the officer class and had to look for a job.
To behave like a caddie, which simply became cad, was to behave in a manner unbecoming of a gentleman.
Incidentally, this is also the origin of the term caddie for the person who carries your golf equipment.
“That guy is such a cad, he stood-up my friend and never even apologized!”.