The word "collateral" comes from the Latin "col-", with + "latus", side = with the side, situated at the side. (The prefix "col-" is a variant form of "com-" meaning "with" that is used before an "l".) Hence, collateral means accessory, subordinate, secondary, serving to support or reinforce.
In the sense of being on the side and in the sense of serving to support or reinforce, there are collateral ligaments outside of the knee joint. The collateral ligaments help provide stability and strength to the knee joint: