means A solid structure of stone, wood, or concrete built out into the water to protect a harbour, guide a current, or serve as a landing place.

from From the French 'jetée,' meaning 'a thing thrown out' — the past participle of 'jeter,' to throw, which descends from Latin 'iactare,' to hurl or fling. So a jetty is literally something flung out into the sea, a cousin of words like 'jet,' 'eject,' and 'projectile,' all sharing that same Latin sense of throwing.