The police found a thumb-print on the doorknob, which will serve as crucial evidence.

thumb-print 是由 thumb（拇指）和 print（印迹）组成的复合名词，字面意思是"拇指留下的印迹"，即"拇指印"。thumb 指"大拇指"，print 指"印记、痕迹"，合起来特指手指留下的独特纹路印记。