The word "Ros" means anger.
Ros Na Kahu Sang Karau - Don't be angry with anyone
All anger rises from desires. And according to many eastern philosophies all desires lead to -- through anger and other negative emotions -- unhappiness. Guru Nanak claims there is one exception. He says, "The hunger of truth (or oneness) satisfies all hungers and consumes all sorrows" - Sache naam ki Laage Bhookh, Ut Bhookhai khai chaliyeh dookh."
So, there is legitimate anger and that is anger towards actions that stray away from oneness. Here is an example from Babarvani. Guru Nanak affirms that one should be concerned and angered about atrocities:
ਕਰਤਾ ਤੂੰ ਸਭਨਾ ਕਾ ਸੋਈ ॥
Karathaa Thoon Sabhanaa Kaa Soee ||
O Creator Lord, You are the Master of all.
ਜੇ ਸਕਤਾ ਸਕਤੇ ਕਉ ਮਾਰੇ ਤਾ ਮਨਿ ਰੋਸੁ ਨ ਹੋਈ ॥੧॥ ਰਹਾਉ ॥
Jae Sakathaa Sakathae Ko Maarae Thaa Man Ros N Hoee ||1|| Rehaao ||
If some powerful man strikes out against another man, then no one feels any grief in their mind. ||1||Pause||