expression
don't push yourself too hard; don't force yourself
A colloquial fixed expression used to caution someone against overexertion or unreasonable effort.
まだ病み上がりなんだから、無理は禁物だよ。
You're still recovering from your illness, so don't push yourself too hard.
徹夜続きで疲れてるなら、無理は禁物です。
If you're tired from pulling all-nighters, you shouldn't force yourself.
A set phrase combining 無理 (unreasonableness, overdoing) with 禁物 (something forbidden or to be avoided). The exact origin is uncertain, but it follows the common pattern ～は禁物 used to warn against a particular action or state.