musar

שְׁמַ֣ע בְּ֭נִי מוּסַ֣ר אָבִ֑יךָ וְאַל־תִּ֝טֹּ֗שׁ תּוֹרַ֥ת אִמֶּֽךָ׃

My son, heed the discipline (musar) of your father,
And do not forsake the instruction (torah) of your mother; 

Rashi:

Hearken, my son, to the discipline of your father What the Holy One, blessed be He, gave Moses in writing and orally.

your mother Heb. אמך [like אמתך], your nation, the nation of Israel, as in (Ezek. 19:2): “What a lioness was your mother [meaning your nation]!” These are the words of the Scribes, which they innovated and added and made safeguards for the Torah.

Thus musar is the written Torah, toras imecha is the words of the rabbis.

How can musar here mean chewing somebody out? It's compared to Torah.

People confuse tochacha (rebuke) with musar. To give somebody musar should only mean instruction, englightenment. 

rebuke is something else.

don't fear the word musar.