Digital Companion to C. S. Peirce
1903 | Syllabus: Nomenclature and Division of Triadic Relations, as far as they are determined | EP 2:291

A Sinsign (where the syllable sin is taken as meaning “being only once,” as in single, simple, Latin semel, etc.) is an actual existent thing or event which is a sign. It can only be so through its qualities; so that it involves a qualisign, or rather, several qualisigns. But these qualisigns are of a peculiar kind and only form a sign through being actually embodied.

1904 | Letters to Lady Welby | SS 32

As it is in itself, a sign is either of the nature of an appearance, when I call it a qualisign; or secondly, it is an individual object or event, when I call it a sinsign (the syllable sin being the first sillable [sic] of semel, simul, singular, etc); or thirdly, it is of the nature of a general type, when I call it a legisign.

1904 | Firstness, Secondness, Thirdness, and the Reducibility of Fourthness [R] | MS [R] 914:6

…in the first place a sign may, in its own firstness, either be a mere idea or quality of feeling, or it may be a ‘sinsign’, that is, an individual existent (and P. holds, with Hegel, that existence consists in the blind reaction of the existent with the rest of the universe in which it exists), or it may (like a word) be a general type (‘legisign’) to which existents may conform.

1905 [c.] | The Basis of Pragmaticism | MS [R] 284:59

A sinsign is a definite individual existent which is significant because of the circumstances of its existence, which are either significant or include the very object denoted.