'socialness' is formed in Modern English from the adjective 'social' + the suffix '-ness' (a noun-forming suffix meaning 'state or quality'). 'Social' ultimately comes from Latin 'sociālis', from 'socius' meaning 'companion, ally'.

'social' entered English via Middle French/Latin influence from Latin 'sociālis'; English later added the native suffix '-ness' (from Old English '-ness') to form 'socialness'.

Originally related to companionship ('companion, ally'), the root developed into an adjective meaning 'relating to companionship or society'; adding '-ness' produced the noun meaning 'the state or quality of being social,' which has remained fairly stable.