To understand what it means, we need more information.
Can you provide some context?
For example:
* Where did you hear this phrase? (A book, a movie, a conversation?)
* What was the situation? (Was it a joke, a serious statement, or something else?)
* Who said it? (Someone you know, a character in a story, etc.)
With more context, I can help you understand the meaning of "scaly wag."