'picky-ness' derives from the adjective 'picky', which was formed from the verb 'pick' + the adjectival suffix '-y' and the noun-forming suffix '-ness'.

'pick' (Middle English 'piken', meaning to peck or choose) → adjective 'picky' (late 19th century, meaning excessively particular) → noun 'pickiness' / variant 'picky-ness'.

Originally 'pick' referred to pecking or picking out; over time the sense of 'choosing' developed, and 'picky' came to mean 'excessively selective,' which is the meaning carried into 'pickiness'/'picky-ness'.