大概
たいがい · taigai · JLPT N1 · adverb
in general
大 (big) + 概 (general outline) = the big outline, so: in general, mainly
大 means big and 概 means general or outline. Together they point to the broad outline of something, which gives the sense of in general or mainly. The same word is also used to mean roughly or probably.
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.