The bank requires customers to provide a thumb-impression on important documents as an additional security measure.

thumb 是名词，意思是"拇指"。 impression 是名词，来自动词 impress（压印、留下印记），指"压印留下的痕迹"。 thumb-impression 指用拇指按压留下的印记，即"拇指印"，常用于身份验证或文件签署。