SLIGHT spoiler stuff for things happening in the future of the AU, but I don't think its super telling.
Translation: "Wait" & "It Reeks Of Zim!"
Along with other Irken head canons I also liked the one where antennae were used for smell, and marking anything. Say, Irkens might use it let other Irkens know which Sir-unit belongs to who, who's uniform belongs to who, (some of them look exactly the same) or even on captured slave from conquered alien races. It's pretty much an common identifier.
Zim at this point has a notorious reputation for bringing destruction and misfortune everywhere he goes, and other aliens, especially Irkens, know this. It causes a...situation here and there.
Dib can understand Irken barely, so all he understood was "it reeks". Why the marking on Dib wasn't entirely 100% necessary....Zim has reasons he will not elaborate. Y'all know.