The Collaborative International Dictionary of English v.0.48:

Idiograph \Id"i*o*graph\ ([i^]d"[i^]*[-o]*gr[.a]f), n. [Gr. ?
   autographic; 'i`dios one's own + gra`fein to write.]
   A mark or signature peculiar to an individual; a trade-mark.
   [1913 Webster] Idiographic