'lactase-active' is formed in English by combining 'lactase' and 'active'. 'Lactase' comes from New Latin 'lactase', itself built from Latin 'lac, lact-' meaning 'milk' plus the enzyme-forming suffix '-ase'. 'Active' comes from Latin 'activus' (via Old French/Latin) meaning 'doing' or 'working'.

'Lactase' was coined in the late 19th/early 20th century as the systematic name for the enzyme that breaks down milk sugar, adopting the suffix '-ase' used for enzymes (derived in part from French enzyme-naming like 'diastase'). 'Active' entered English from Latin 'activus' through Old French and Middle English; combining terms by hyphenation to form descriptive compounds (e.g., 'lactase-active') is a modern English formation.

Initially, 'lactase' denoted the enzyme itself; 'lactase-active' later emerged to describe organisms, tissues, or samples that exhibit lactase enzymatic activity. Its current meaning emphasizes the presence of enzyme activity rather than naming the enzyme alone.