Several hands dropping a share each into the same bucket. What comes out belongs to all of them: no single share decides it, but take one away and the result changes.
The word is built from con- (together) and the tribute family (a share handed over). Together: each side hands over a share.
這個字可以拆成 con-（一起）和 tribute 那一族的「交出去的一份」。合起來就是「各出一份，一起湊」。這正好說明它為什麼總帶著「之一」的味道。
Wiktionary entry supplied in this batch's evidence packet (source_ref kaikki:3f6f69a190036ae4:1413698669): from earlier contributary, from Middle English contributarie, from con- + tributarie after Middle French contributaire.
Two branches
丟進去的是原因
桶子裡裝的是各種原因，結果是它們一起造成的。
adjective · C1
Helping to cause a particular result, but not being the only cause of it. This word goes in front of the noun, as in a contributory factor or a contributory cause, and it is often used about something that went wrong.
促成的、原因之一的
丟進去的是錢
同一個桶子，只是這次大家丟的是每個月的錢。
adjective · C1
A contributory pension plan, insurance plan or savings plan is one that the workers pay money into as well as the company they work for.
勞資共同提撥的