Due to his lack of experience, his application was deemed unsuitable for the senior position.

unsuitable 是由形容词 suitable（合适的）加上否定前缀 un- 构成的形容词，表示"不合适的"。un- 是常见的否定前缀，表示"不、非"的意思。suitable 来自动词 suit（适合）加上形容词后缀 -able（能够...的）。