'clammy' originates from a dialectal English word 'clam' (attested in Middle/Early Modern English), where 'clam' meant 'cold' or 'damp'. The adjectival suffix '-y' was added to form 'clammy'.

'clam' (dialect) developed into the adjective 'clammy' by the addition of the suffix '-y' in Early Modern English, eventually becoming the standard modern English word 'clammy'.

Initially it meant 'cold and damp'; over time it has come to mean specifically 'unpleasantly damp and slightly sticky' and is also used figuratively for 'cold/unfriendly'.