A male cat was originally a ram, not a tom. Contrary to wide belief, the origin of the term is not a contraction of the moderately popular feline name Thomas.
The credit is due to one Tom the Cat, the handsome hero of a racy book titled The Life and Adventures of a Cat published anonymously in 1760. The story was so popular that the name caught on, and has stayed with us for almost two and a half centuries now.
Even though the origin of it is so obscure, use of the term tomcat (or tom cat) still implies a healthy sex life, particularly when transferred to humans. The unfortunate Gregory Poirier movie Tomcats (2001) drew crass humour from this fact, and sadly mutilated what used to be a nice little phrase.
A man described as a tomcat is likely to have a strongly masculine sexual identity, and a tomboy is a woman (especially a young woman or girl) who also behaves in a way culturally identified with masculinity, though it need not imply sexual identity or sexuality.