祖母
そぼ · sobo · JLPT N4 · noun
grandma
👵 祖母 = ancestor (祖) + mother (母): the mother of your line
祖 means ancestor or forefather, 母 means mother, so 祖母 is literally ancestor-mother. It pairs with 祖父 (sofu, ancestor-father = grandfather). 祖母 is the humble, formal word for your own grandmother; おばあさん is the everyday one.
母 — pictograph of kneeling woman with two breast dots
母 is a pictograph derived from 女 (woman) with two added dots representing breasts, marking a nursing mother. The earliest oracle bone forms show a kneeling figure with prominent dots on the chest. The meaning has stayed remarkably stable, denoting 'mother' and by extension 'female' or 'source/origin'.