As verbs the difference between stiff and strengthen
is that
stiff is to fail to pay that which one owes (implicitly or explicitly) to another, especially by departing hastily while
strengthen is (
lb) to make strong or stronger; to add strength to; to increase the strength of; to fortify; to reinforce.
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 .