"Collateral" is a word in LAW AND LEGAL, ENGLISH

By the side; at the slde; attached upon the side. Not llneal, but upon a parallel or diverging line. Additional or auxlllary; supplementary; cooperating

Related to, but not strictly a part of, the main thing
or matter under consideration; hence, subordinate; not chief or
principal; as, collateral interest; collateral issues.

Acting in an indirect way.

Descending from the same stock or ancestor, but not in
the same line or branch or one from the other; -- opposed to lineal.

Tending toward the same conclusion or result as
something else; additional; as, collateral evidence.

Coming from, being on, or directed toward, the side;
as, collateral pressure.

A collateral relative.

Collateral security; that which is pledged or deposited
as collateral security.