It has to do with a book called The Life and Adventures of a Cat published in 1760. This book featured a feline protagonist named Tom, who was a promiscuous male cat. The name Tom or Tomcat has been widely used ever since.
Tomcats are cats that have several secondary sexual characteristics that make them look different from their female counterparts. This term refers to a male cat that’s still intact and sexually mature.
Sexually mature male cats will have large, fat jowls. These cats will roam in search of females, and are more likely to spray to mark their territory. These behaviors can be resolved by neutering the cat.