What is the comparative and superlative form of lonesome?

The word "lonesome" is already in the superlative form. There's no more "loneliness" to be had than being "lonesome".

However, if you want to express different degrees of loneliness, you could use these options:

* Comparative: more lonely

* Superlative: most lonely

These options are more commonly used when talking about general loneliness, rather than the specific feeling of "lonesome".