'boyishness' is formed in modern English by combining the noun 'boy' with the adjectival suffix '-ish' and the nominalizing suffix '-ness' to mean 'the quality of being boyish.'

'boy' comes from Middle English 'boy(e)' (also 'boi'), originally used for a male servant or a young male; over time it came to mean 'male child' or 'young male', and the modern compound 'boyishness' developed by adding '-ish' and '-ness' in later English.

Originally 'boy' could denote a male servant or young male; through history it narrowed to mean a young male/child, and 'boyishness' came to denote qualities associated with that youth (playfulness, naivety, immaturity).