This dead dog - See the marginal reference and 2 Samuel 9:8 note.
Go over - The ravine, possibly with a stream of water 2 Samuel 17:20, which lay between them and Shimei.
Related Commentaries of 2 Samuel 16:9
2 Samuel 16:9
9 Then said Abishai the son of Zeruiah unto the king, Why should this dead dog curse my lord the king? let me go over, I pray thee, and take off his head.