伯母
おば · oba · JLPT N2 · noun
aunt
伯母 = 伯 (eldest) + 母 (mother): a senior mother, older than your parent
伯 means eldest or senior, marking an aunt who is older than your parent. Pair it with 伯父 (uncle) for the older-side set. This is the humble term for your own aunt.
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'.