ご多忙中[gotabouchuu] in the midst of your work (often used as a formal phrase in business letters, etc.)
多忙[tabou] busy
忙がしい[sewashii] busy
繁忙[hanbou] pressure of business
忙殺[bousatsu] being extremely busy

忙しげ[sewashige] looking busy
気忙しい[kizewashii] restless
忙中[bouchuu] (in the midst of) busyness
心忙しい[kokorozewashii] restless
仕事で忙しい[shigotodeisogashii] busy with one's work