'antitarnishing' is formed from the prefix 'anti-' (from Greek 'anti-' meaning 'against') combined with 'tarnish' (modern English) to mean 'against tarnish' or 'preventing tarnish'.

'tarnish' comes from Middle English (from Old French 'tarnir' or 'tarnir') meaning 'to make dark or discolored'; the English prefix 'anti-' is from Greek 'anti-' meaning 'against'. These elements combined in modern English to form compounds like 'antitarnish' and the derived '-ing' form 'antitarnishing'.

Originally the parts literally meant 'against' + 'make dark'; over time the compound has come to refer specifically to treatments, coatings, or properties that prevent discoloration (i.e., 'prevent tarnish').