赤坂
Akasaka
The word is made of 2 kanji:
赤 aka : red
坂 saka : hill, slope
There are two explanations. A third explanation is just a combination of these two.
The first explanation is that the slope had ruddy clay that gave it a distinctive look.
The second explanation is that the hill was formerly referred to as Akanesaka or Akaneyama (written赤根山 Akaneyama or 赤根坂 Akanesaka or 茜坂 Akanesaka). 茜 akane Japanese madder or 赤根 akane Japanese madder (literally “red root”), is a plant used to make a brilliant red dye (from its root). Supposedly the area was famous for this plant and it was easily collected there before the arrival of the Tokugawa. Over time, the pronunciation became slurred and Akanesaka turned to Akasaka. The 赤根坂 Akanesaka variant could easily be reduced to 赤坂 Akasaka.
The third theory is a mixture of those. It points out that the roots of the Japanese madder, being used for making red dye, naturally turned the dirt and clay of the slope red. Therefor the hill was known for a plant already associated with red and a hill that had reddish dirt.
