腕を上げる。

The Japanese expression '腕を上げる' (ude o ageru) literally translates to 'to raise one's arm.' Its figurative meaning, 'to get better at something' or 'to improve one's ability,' likely stems from the historical and cultural context of martial arts and other skills where demonstrating proficiency often involved specific arm movements or postures. In many traditional Japanese disciplines, such as swordsmanship (kendo), archery (kyudo), or even calligraphy (shodo), the control and elevation of the arm are fundamental to executing techniques correctly and powerfully. As a practitioner's skill develops, their ability to control and raise their arm with precision and strength improves, signifying their progress. Another possible origin lies in the idea of a laborer or artisan becoming more adept. When someone becomes more skilled at their craft, they can perform tasks with greater ease and efficiency, perhaps metaphorically 'lifting their arm' with less strain or more confidence. The phrase would then have evolved to generally describe an improvement in any skill or ability, not just those involving physical arm movements. Over time, '腕を上げる' became a common idiom to express enhancement in a broad range of capabilities, from sports and academic subjects to professional skills. It encapsulates the idea of honing one's abilities through practice and dedication, much like a martial artist diligently refines their techniques until they can execute them flawlessly with a raised arm.