His retirement fund has accumulated a lot of interest over the past five years.

accumulated 是动词 accumulate 的过去分词形式。accumulate 由前缀 ac-（表示"向、加强"）和词根 cumulate（堆积）组成，意思是"逐渐增加、积累"。这里表示退休金账户中的利息不断累积增加的状态。