A neg is a backhanded insult, poorly disguised as a compliment.

For example: "For being an engineer, you are a good designer"

This is an indirect way of saying "your are not a good designer". This criticism is not constructive, and implies that it is impossible to be skilled at two things.

Instead, a more constructive communication is to give direct feedback. For example: "your designs would be better if you talked to more non-technical users"