'mournfulness' derives from the adjective 'mournful' + the noun-forming suffix '-ness'; 'mournful' is from the verb 'mourn' + the adjective suffix '-ful'. The verb 'mourn' itself originates from Old English 'murnan', meaning 'to grieve'.

'mourn' changed from Old English 'murnan' to Middle English 'mournen' (and forms like 'mournen'), then became modern English 'mourn'; from this came 'mournful' and later the nominal 'mournfulness' by adding '-ness'.

Initially associated with the action 'to grieve' (for 'mourn'), the sense extended to an adjectival character 'mournful' and then to the abstract noun 'mournfulness' meaning the state or quality of sadness/solemnity.