Used to describe something or someone as being superior, or
excellent.
So superior that it is incapable of being changed from its level of 'greatness', hence "solid"
Like a solid
foundation, what is being described as "solid" is so
excellent that it cannot be made less awesome.
"Son, those new
two-tone kicks are indeed 'solid'!"
"Hey Joe, that new guitar riff sure is 'solid'"
"
Check out Captain Cookout's new album! Its 'SOLID'!"