'portside' is a compound formed in modern English from 'port' + 'side'. 'Port' in this sense ultimately comes from Old French 'port' (from Latin 'portus') meaning 'harbour', while 'side' comes from Old English 'sīd' meaning 'side'.

The term for the left side of a vessel was historically 'larboard'; in the 19th century (officially adopted by the Royal Navy around the 1840s) 'port' replaced 'larboard' to avoid confusion with 'starboard', and compound forms such as 'portside' became the standard modern usage.

Originally 'port' primarily meant 'harbour' (from Latin 'portus'); over time it came to be used for the side of a ship that faces the harbour when moored, and 'portside' evolved to mean the left side of a vessel when facing forward.