The verse states (Ruth 4:1):
וּבֹעַז עָלָה הַשַּׁעַר, וַיֵּשֶׁב שָׁם, וְהִנֵּה הַגֹּאֵל עֹבֵר אֲשֶׁר דִּבֶּר-בֹּעַז, וַיֹּאמֶר סוּרָה שְׁבָה-פֹּה פְּלֹנִי אַלְמֹנִי; וַיָּסַר, וַיֵּשֵׁב
Now Boaz went up to the gate, and sat him down there; and, behold, the near kinsman of whom Boaz spoke came by; unto whom he said: 'Ho, such a one! turn aside, sit down here.' And he turned aside, and sat down.
Why is everyone so convinced that Ploni Almoni is meant to be a placeholder of sorts and not actually his name? And if it is a placeholder, then what is its root? P-L-N? What does that mean?