'pressure-varying' originates from Modern English as a compound of the noun 'pressure' and the present-participle adjective 'varying'.

'pressure' comes into English via Old French/Norman (cf. Old French 'pression') from Latin 'pressura' (from 'premere', 'to press'); 'varying' is formed from 'vary', which derives from Latin 'variare' (from 'varius', 'different') through Old French 'varier' into Middle English 'varien/varien'. These components were combined in Modern English to form the compound adjective 'pressure-varying'.

Individually, 'pressure' originally referred to the act of pressing and 'vary' meant 'to make different'; together in Modern English the compound came to mean 'undergoing changes in pressure' (a technical/ descriptive sense).