習慣
しゅうかん · shuukan · JLPT N4 · noun
custom
習慣 = learn/practise (習) + get used to (慣): practised until it is the norm
習 means to learn by repetition, as in 練習 (practice). 慣 means to grow accustomed, as in 慣れる (get used to). A custom is what a people has repeated so long it became second nature. Sound hook: shuukan also means week (週間), and customs repeat weekly.