broker

Broker comes from Middle English broker, brokour, from Anglo-Norman brocour meaning 'small trader,' which traces back to Old Dutch *brokere, 'one who determines the usages of trade, manager.' That in turn comes from a Germanic word for use or custom, broke, bruyck, breuck, rooted in Proto-Indo-European *bʰruHg-, 'to use, enjoy.'

The word began as a description of someone who knew the customs and usages of trade — essentially a specialist in how deals were done — and settled into naming the go-between who handles a trade for others.

It's built from the same root as brook, in the old sense of 'to use or tolerate,' since a broker was literally one who 'used' or handled trade on someone else's behalf.