'ignorable' originates from Latin via Old French/Anglo-Norman, specifically from the Latin verb 'ignorare', where the prefix 'in-' meant 'not' and the root related to 'gnō-' (from Latin 'gnoscere') meant 'to know'; the adjective was formed by adding the suffix '-able' (from Latin '-abilis') meaning 'able to be'.

'ignore' changed from Old French/Anglo-Norman word 'ignorer' (from Latin 'ignorare') and eventually became the modern English verb 'ignore'; 'ignorable' was later formed from this English verb by adding the adjectival suffix '-able'.

Initially the Latin verb meant 'not to know' or 'be unaware of'; over time in English it developed the sense 'to deliberately not pay attention to' or 'to disregard', and 'ignorable' came to mean 'able to be ignored' or 'not important enough to notice'.