ばかげる: looks/thought stupid
ばからしい: feels stupid, nonsensical
While there are some different nuances, which I shall not attempt to elucidate here, the two are as much used as the simplest adjective ばかだ is. They are not slang words.
I think baka
is of unknown etymology (like so many words in this language) but it seems to have been long considered of Chinese origin.
The following passage from 史記李斯列傳
was the supposed source of the expression:
After The First Emperor, Qin dynasty was usurped by The Second Emperor, who was controled by Zhao Gao, the eunach secretariat. Zhao Gao presented a deer to the emperor, saying that it was a horse. The puppet emperor asked his attendants what they see. Afraid of Zhao Gao's revenge, they all replied that it was a horse.
This, however, cannot be the authentic etymology since the kanji representation of the word as per this etymology, 馬鹿, should be read baroku
if it were assumed of Chinese origin. In other words, 鹿 as ka
is a Japanese reading (kun-yomi).
Etymologically speaking, 馬鹿 is not the correct kanji representation for the reasons I have presented. This is by no means to suggest that we should not use this kanji representation. I consider this folk etymology has been with the word itself long enough to be part of the meaning. We can, therefore, be magnanimous to use 馬鹿な.