'lactonic' originates from modern English formation based on 'lactone' + the adjectival suffix '-ic', ultimately from New Latin/Neo-Latin 'lacton-' (from Latin 'lac, lact-' meaning 'milk').

'lactonic' developed from the chemical term 'lactone' (coined in the 19th century for certain cyclic esters), which itself was formed from Latin 'lac, lact-' ('milk') via Neo-Latin 'lacton-'; the adjective was created by adding '-ic' to form 'lactonic'.

Originally the root 'lac/lact-' referred directly to 'milk'; with the rise of organic chemistry it came to name specific compounds ('lactone') and 'lactonic' now primarily means 'relating to a lactone', though an older sense 'milk-like' can remain in rare usages.