כְּרֵֽעַ־כְּאָ֣ח לִ֭י הִתְהַלָּ֑כְתִּי כַּאֲבֶל־אֵ֝֗ם קֹדֵ֥ר שַׁחֽוֹתִי׃
I went about as though it had been my friend or my brother; I bowed down mournful, as one that mourneth for his mother.
Rashi on Psalms
I walked about as though it were a friend or as though it were my own brother As though it were my brother or my friend; I walked about depressed because of their distress.
Rashi on Psalms
as the mourning of a mother As a son who mourns for his mother or as a mother who mourns for her son.