'mysticalness' originates from Modern English by adding the suffix '-ness' to 'mystical'; 'mystical' comes from Latin 'mysticus' (via Old French 'mystique'), ultimately from Greek 'mustikos' related to 'mysterion' meaning 'mystery'.

'mysticalness' formed in Modern English from 'mystical' + '-ness'; 'mystical' changed from Old French 'mystique' and Latin 'mysticus', which derived from Greek 'mysterion' and 'mustikos', and eventually became the modern English 'mystical'.

Initially the root terms referred to 'mystery' or 'things of secret rite/knowledge'; over time the sense broadened to 'relating to spiritual or transcendent experience' and 'mysticalness' now denotes the quality of being mystical.