大した
たいした · taishita · JLPT N3 · na-adj
significant
大した = 大 big + した did: something that did big is considerable
Built from 大 (big) plus した, the past form of する (to do). It is most common in the negative: 大したことない means no big deal, literally nothing that amounts to much.
大 — pictograph of a person with arms outstretched
大 is a pictograph showing a person standing with arms and legs spread wide. The outstretched arms suggest 'big' or 'great', extending from the literal image of a large stance to abstract meanings like important or grand. It is one of the oldest characters, found on oracle bones.