'antilactase' is formed in modern scientific English by combining the prefix 'anti-' (from Greek 'anti', meaning 'against') with 'lactase' (the enzyme name).

'lactase' itself is built from Latin 'lac, lact-' (from 'lac, lactis' = 'milk') + the enzyme-forming suffix '-ase' (a 19th-century biochemical coinage). The compound 'antilactase' is a modern coinage combining 'anti-' + 'lactase'.

Initially the elements meant 'against' and 'milk/enzyme of milk' respectively; the combined scientific coinage came to mean specifically 'against the enzyme lactase' (i.e., an inhibitor or antibody to lactase).