Decreased Upper Arm
Scholars have said that if one dreams of a decrease in their upper arm (deltoid region), it may indicate a calamity befalling their brother or adult son, proportionate to the decrease. They also mentioned that the forearms in interpretation may symbolize two close relatives or friends from whom the dreamer benefits and relies upon.
Per Ibn Sirin's Muntakhab al-Kalam fi Tafsir al-Ahlam