'soothingness' ultimately comes from the verb 'soothe', which in turn comes from Old English 'sōthian' (related to 'sōth' meaning 'true').

Old English 'sōthian' ('to be true') developed into Middle English forms such as 'soothen'/'sothen' meaning 'to comfort/console' and later into modern English 'soothe'; the noun was formed by adding the adjectival '-ing' (soothing) and the nominalizing suffix '-ness' to create 'soothingness'.

Originally associated with 'truth' or 'being true', the verb shifted in Middle English to mean 'to console or relieve', and the modern meaning centers on 'producing comfort or calm'—hence 'soothingness' = 'the quality of calming or comforting'.