His uncommunicative personality made it difficult for coworkers to understand his true thoughts.

uncommunicative 是由前缀 un-（表示"不"）和形容词 communicative（善于交流的）组成的复合形容词。communicative 来自动词 communicate（交流），加上形容词后缀 -ative 表示"具有...性质的"。 uncommunicative 字面意思是"不善于交流的"，引申为"沉默寡言的、不爱说话的"。