'easygoingness' originates from Modern English, specifically formed by the adjective 'easygoing' plus the nominalizing suffix '-ness'; 'easygoing' itself is formed from 'easy' + 'going'. 'easy' ultimately comes from Old English 'ēaðe', where 'ēaðe' meant 'easy, simple'.

'easy' comes from Old English 'ēaðe'; 'go' comes from Old English 'gān' (to go); the suffix '-ness' comes from Old English '-nes(s)e'. The compound sense 'easygoing' arose in Modern English (early 20th century), and adding '-ness' produced 'easygoingness'.

Initially the component words meant 'easy' (not difficult) and 'going' (movement); over time the compound 'easygoing' developed the sense 'relaxed or tolerant', and 'easygoingness' came to mean the state or quality of being relaxed/tolerant.