His guess was completely off-base and didn't match the facts.

off-base 是由介词 off（偏离）和名词 base（基础）组成的复合形容词，字面意思是"偏离基础的"，引申为"错误的、不符合事实的"。这个短语常用于描述与实际情况不符的猜测或判断。