word-forming element denoting "kinship one degree further removed," early 15c. (in great uncle), from great (adj.), based on similar use of French grand (see grand-). An Old English way of saying "great-grandfather" was þridda fæder, literally "third father;" in early Middle English furþur ealdefader was used (12c.).
grandmother n.
early 15c., from grand- + mother (n.1), probably on analogy of French grand-mère. Replaced earlier grandame (c. 1200) and Old English ealdemodor.