'instinctual' originates from Latin via English 'instinct', specifically the Latin word 'instinctus', where the root (from the verb often cited as 'instinguere' or similar) meant 'to incite' or 'to urge on'.

'instinctus' passed into Old French as 'instinct' and then into Middle English as 'instinct'; the modern adjective 'instinctual' was formed in English by adding the adjectival suffix '-ual' to 'instinct'.

Initially, the related Latin term referred to a prompting or urging ('a prompting/urge'); over time the meaning shifted into English to denote an inborn tendency or natural impulse, and 'instinctual' now means 'relating to instinct' or 'acting on instinct'.