Thumbing through a dictionary to find synonyms for "thumbprint" could be an interesting exercise! Here are a few suggestions: imprint, mark, impression, stamp, seal, fingerprint, signature, autograph, endorsement, symbol, monogram, logo, emblem, insignia, badge, crest, device, motif. Depending on the context, some of these words might be more appropriate than others. For example, "fingerprint" would typically refer to the identifying pattern of ridges on the tip of a person's thumb (or another extremity). "Logo" or "crest" might describe a stylized image created by pressing one's thumb onto a surface as a form of branding or decoration.