'grandmotherly' originates from English, specifically the word 'grandmother' plus the suffix '-ly', where 'grand' comes (via Old French 'grand') meaning 'large' or 'great' (used in kinship to indicate one generation removed), 'mother' comes from Old English 'mōdor' meaning 'female parent', and the suffix '-ly' (from Old English '-lic') meant 'like' or 'having the quality of'.

'grandmotherly' changed from the Middle English formation involving 'grandmoder' (a compound of Old French 'grand' + Old English 'mōdor') into the Modern English noun 'grandmother'; the adjective was then formed by adding the suffix '-ly', producing the modern English 'grandmotherly'.

Initially, it meant 'pertaining to or characteristic of a grandmother', and over time this basic sense has largely remained, though it has also acquired connotations of being 'old-fashioned' or 'overly sentimental' in some contexts.