Solidly is a related term of stiff.
As an adverb solidly
is in a solid or firm manner.
As an adjective stiff is
of an object, rigid, hard to bend, inflexible.
As a noun stiff is
an average person, usually male, of no particular distinction, skill, or education, often a
working stiff''''' or ''lucky '''stiff .
As a verb stiff is
to fail to pay that which one owes (implicitly or explicitly) to another, especially by departing hastily.