“avenger of blood.” A member of one’s family or clan that would kill anyone who killed someone in the family (see commentary on Num. 35:19).
“not one hair of your son will fall to the earth.” In its historical context, this is ironic. David says of this make-believe person that not one hair of his head would fall to the earth when the real story and ruse was about Absalom, who had beautiful, abundant hair (2 Sam. 14:25-26), which got stuck in a tree and who died without a hair touching the earth (2 Sam. 18:9-17).