thumb print; nail impression
Refers to a mark made by pressing a fingernail or thumb into a surface, often used historically as a signature or seal substitute.
Old documents sometimes have a thumb print pressed instead of a signature.
Compound of 爪 (つめ, 'nail') and 印 (いん, 'mark' or 'seal'). The exact historical derivation is uncertain, but the word straightforwardly describes a nail impression.