'peninsula' originates from Latin, specifically the word 'paeninsula' (or 'paene' + 'insula'), where 'paene' meant 'almost' and 'insula' meant 'island'.

'peninsula' passed into Medieval/Modern Latin as 'peninsula' (also attested as 'paeninsula'), entered Old French/Anglo-Norman forms such as 'peninsule', and was adopted into Middle English as 'peninsula', eventually becoming the modern English word 'peninsula'.

Initially it literally meant 'almost-island' ('almost an island'), and over time it came to denote the geographic feature we use today: land projecting into water but still joined to the mainland.