'irrepressibly' originates from English, formed by adding the adverbial suffix '-ly' to 'irrepressible'. 'Irrepressible' is built from the negative prefix 'ir-' + 'repressible', where 'repress' derives from Latin elements meaning 'again/back' and 'to press'.

'repress' comes from Latin 'reprimere' meaning 'to press back'; through Late Latin and Middle French the verb entered Middle English as 'repress'. From this came the adjective 'repressible' (able to be repressed), which took the negative prefix to become 'irrepressible' in Early Modern English; the adverb 'irrepressibly' was formed later by adding '-ly'.

Initially related to the idea of 'pressing back' or 'suppressing' (from 'reprimere'), the negative form came to mean 'not able to be suppressed', and this core sense remains in the modern adverb 'irrepressibly' ('in a way that cannot be suppressed').