'underpinned' derives from the English verb 'underpin', a compound of 'under' + 'pin', where 'under' meant 'below' and 'pin' (as a verb) meant 'to fasten, secure, or support'.

'underpin' developed in modern English as a compound (literally 'to prop or fasten from below') and was used in both literal building contexts and later in figurative senses; the past form 'underpinned' is the regular past/past-participle of this verb.

Initially it was used mainly for literal, physical support ('to prop up from beneath'); over time it gained frequent figurative use meaning 'to provide a basis, justification, or support (for an idea, system, argument)'.