'reassuringness' originates from English, specifically formed from the verb 'reassure' plus the present-participial/adjectival suffix '-ing' and the noun-forming suffix '-ness', where the element 're-' indicates 'again' or 'back' and 'assure' meant 'make safe or certain'.

'reassure' changed from Middle English/Old French roots such as Old French 'asseurer' (to make safe), which in turn comes from Latin elements related to 'securus' ('secure' or 'free from care'); the modern English formation 'reassuringness' is created by adding '-ing' and '-ness' to that base.

Initially the Latin root conveyed the idea 'free from care' or 'secure'; over time, through Old French and Middle English, the sense shifted to 'make certain or give confidence', and the modern compound 'reassuringness' denotes the quality of providing reassurance or comfort.