Where did the different names for the "priestly blessings" come from?
Duchaning , Birkas Kohanim, Nesiat Kapayim etc..
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According to Rabbi Yirmiyohu Kaganoff, duchaning has Aramaic roots and refers to the platform that is in front of the Aron Kodesh.
The Duchan, דוכן, which can for example be found in the Mishnah in Middos 2:6, is referring to a three-staired platform
there was a step a cubit high on which a platform was placed, and it had three steps each of half a cubit in height.
The Duchan is numbered "12" in the following picture (Akiva Males from Torah Musings):
Rabbi Akiva Males has an interesting explanation on why Birkas Kohanim is also referred to as duchan. He writes:
In order for Kohanim to enter the Ezras Kohanim to engage in any aspect of the Avodah, it was necessary to first ascend the Duchan by way of the Ezras Yisroel.
Nesiat Kapayim, נשיאת כפים, literally means "raising the hands [to bless]". See for example Shulchan Aruch (Orach Chaim 128:1), Taanis 26b. The Midrash (Sifrei Bamidbar 39:1) writes the reason behind nesiat kapayim:
Blessing is likened to ministering. Just as ministering is standing, so, blessing. "Thus shall you bless the children of Israel": with raised hands. You say, with raised hands; but perhaps either with or without raised hands is permitted. It is, therefore, written (Vayikra 9:22) "And Aaron lifted his hands to the people and he blessed them." Just as there, with raised hands, so, here. R. Yonathan says: But perhaps just as there, Rosh Chodesh, offering, and the high-priest, so, here (these elements must obtain)!
My apologies. I did not find a good source describing the origins of the word birkas kohanim. Might add that in later.