Origin and history of wharf

"supported platform or other structure of some size into a river or other body of water, where ships can tie up and load or unload;" late Old English hwearf "shore, bank where ships can tie up," earlier "dam, embankment," from Proto-Germanic *hwarfaz (source also of Middle Low German werf "mole, dam, wharf," German Werft "shipyard, dockyard"); related to Old English hwearfian "to turn," perhaps in a sense implying "busy activity."

This is reconstructed to be from PIE root *kwerp- "to turn, revolve" (source also of Old Norse hverfa "to turn round," German werben "to enlist, solicit, court, woo," Gothic hvairban "to wander," Greek karpos "wrist," Sanskrit surpam "winnowing fan").

Wharf rat is attested from 1812 in reference to the type of rat common on ships and docks, commonly the brown rat,

considered with reference to its being in many places an imported animal, first naturalized in wharves after leaving the ship which brings it, or to the special size, ferocity, or other distinctive character it acquires under the favorable conditions of environment afforded by wharves, shipping, and storehouses. [Century Dictionary]

The extended sense "person who hangs around docks" is recorded from 1836.