pier n.

late Old English, pere, "support of a span of a bridge," from Medieval Latin pera, a word of unknown origin, perhaps from Old North French pire "a breakwater," from Vulgar Latin *petricus, from Latin petra "rock" (see petrous), but OED is against this. Meaning "solid structure in a harbor, used as a landing place for vessels; mole or jetty projecting out to protect vessels from the open sea" is attested from mid-15c.