unimpressionableunimpressionable

Unimpressionable comes from French impressionnable ‘impressionable’, and goes back to Latin premō ‘to press, push, press close or hard, oppress,…’.

1. Latinc. 75 BCE–200 CEpremō¹‘to press, push, press close or hard, oppress, overwhelm’
2. Latinc. 75 BCE–200 CEimprimō¹‘to press, impress’
3. Latinc. 75 BCE–200 CEimpressiōnem¹
4. Frenchc. 1600–presentimpression¹‘an impression, the overall effect of something’
5. Frenchc. 1600–presentimpressionner¹‘to impress, to wow, to awe’
6. Frenchc. 1600–presentimpressionnable¹‘impressionable’
7. English1500–presentimpressionable¹
8. Englishearliest quotation 1914unimpressionable¹

1. ¹ Wiktionary, English unimpressionable
2. Earliest quotation in Wiktionary’s entry: 1914, Ernest Bramah: “Creake, in accordance with some detail of his never-revealed…”.

Earlier history uncertain. See Wiktionary.