Refers to the part of the arm from the shoulder to the elbow.
I do exercises to strengthen my upper arms.
She applied sunscreen to her upper arms.
lower arm
Archaic term for the forearm; modern Japanese uses 前腕 (ぜんわん) or 腕 (うで) for the lower arm. The archaic sense is preserved here for reference.
See also: 一の腕
In old texts, 二の腕 can refer to the forearm.
Literally 'second arm', originally part of a counting system where 一の腕 (first arm) was the upper arm and 二の腕 (second arm) was the forearm. Over time, the meaning shifted to the modern 'upper arm'.