His opinions were often overlooked in major decisions, making him feel a sense of unimportance.

unimportance 是由形容词 important（重要的）加上否定前缀 un- 构成的反义词，表示"不重要"。后缀 -ance 将形容词转化为名词，所以 unimportance 指"不重要、无足轻重的状态"。