I encountered this while reading Astérix. The exchange is as follows, where the druid is a prisoner in a Gothic jail being threatened by his captors who are demanding to see his magic:
Goth: Accepte de montrer ta magie, druide ! Je te couvrirai d'or !
Druide: T'en as un œil !
I'm guessing it's some snappy retort to the effect of "shove it up your #%^&", but I'm not sure what the actual expression is here. En avoir un œil ? When can this be used? What is the exact meaning?