I know 二百五 stands for foolish/stupid and I assume I have to add up 二百五, 三八 and 二 to it to obtain 二百九. Is this right and if so, what does 二 mean and where did it get it's meaning?
Comes from "March" (3, 三) "Eighth" (8, 八):
and then evolved into what it means today (stupid, foolish, batty, gossipy, bitchy). It has no relation to 二百五.
Basically 250 = 半封 which sounds like 半疯 or half-crazy.
tl;dr or don't understand chinese: 1. came from topolect 2. shortening of 250
We Chinese don't say "二百九" but just "二百五" to describe someone is foolish, a typical example is:
Besides we also have "二货"、“二B（Chinese character：屄；Chinese pronuncation: bi）”，This is a offensive saying that is always used to abuse others when you are angry or despise others (very very dirty!), so PLEASE DON'T USE THAT.
"二" can be also used traditionally in the north dialect of China, meaning "foolish". It can be used as an adjective word——e.g: