'undercure' originates from a combination of Old English and Latin (via Old French): the Old English prefix 'under' and Latin 'curare' (through Old French 'curer'), where 'under-' meant 'below' or 'insufficient' and 'curare' meant 'to take care of / to make well.'

'cure' changed from Latin 'curare' to Old French 'curer', then to Middle English forms such as 'curen' and eventually modern English 'cure'; the prefix 'under-' comes from Old English 'under' and combined with 'cure' in Modern English to form compounds such as 'undercure'/'undercured.'

Initially the elements meant 'under' (below/insufficient) + 'to make well or treat'; over time the compound/derivative came to mean specifically 'to treat or process so that the curing is insufficient,' i.e., 'not fully cured.'