שִׁיתָ֣ה יְ֭הוָה שָׁמְרָ֣ה לְפִ֑י נִ֝צְּרָ֗ה עַל־דַּ֥ל שְׂפָתָֽי׃
Set a guard, O LORD, to my mouth; Keep watch at the door of my lips.
Rashi on Psalms
O Lord, place a guard for my mouth when I pray before You, that I should speak poetically, that it should be acceptable.
Rashi on Psalms
the portal of my lips That is the upper lip (like portals of my lips).