To permit his blood/ to be outlawed
He made his life worthless. He permitted his killing. He made it so that it would be permissible to harm him or kill him without receiving any punishment for doing so.
("Rabbi Nachmani said: Because of the name that shed the blood of David, who was called Admoni, who said to Saul, "He is the son of David, he is dead" (1 Samuel 20), his blood is permissible and his wife is permissible to marry another" - Midrash Tehillim (Buber) from Psalm Nob)
Translation
Expression (Hebrew)
הִתִּיר אֶת דָּמוֹ (הֻותַּר דָּמוֹ, דָּמוֹ מֻותָּר)
Definition (Hebrew)
הָפַךְ אֶת חַיָּיו לְחַסְרֵי זְכוּת קִיּוּם. הִתִּיר אֶת הֲרִיגָתוֹ. גָּרַם לְכָךְ שֶׁיְּהֵא מֻתָּר לִפְגֹּעַ בּוֹ אוֹ לְהָרְגוֹ לְלֹא קַבָּלַת כָּל עֹנֶשׁ עַל כָּךְ.
("ר' נַחְמָנִי אָמַר: עַל שֵׁם שֶׁהִתִּיר דָּמוֹ שֶׁל דָּוִד שֶׁנִּקְרָא אַדְמוֹנִי, שֶׁאָמַר לְשָׁאוּל עַל דָּוִד בֶּן מָוֶת הוּא (שְׁמוּאֵל א' כ), דָּמוֹ מֻתָּר וְאִשְׁתּוֹ מֻתֶּרֶת לְהִנָּשֵׂא לְאַחֵר"- מִדְרַשׁ תְּהִלִּים (בּוּבֶּר) מִזְמוֹר נֹב)