The word “port” is straightforward. It’s the left side customarily toward the dock when a ship is in port.
That custom apparently arose because it’s the opposite side to “starboard,” a word that comes from the Old Norse word stjόrnborði (“steering board”). This designated an oversized oar or board fixed to the right aft side and used to control the direction of a ship before rudders were invented. The custom may have arisen to avoid risk of damage to the vulnerable steering mechanism when in port.