expression, godan verb (-ru)
to become stupid from associating with stupid people; to be infected with stupidity
Colloquial expression used humorously or critically when someone seems to have picked up foolish behavior or thinking from others. The metaphor is of stupidity being contagious like a disease.
See also: 移る
あいつと話してると馬鹿がうつるよ。
If you talk to that guy, you'll catch his stupidity.
If you keep doing stuff like that, you'll end up stupid too.