It's related to the -ist suffix which means amongst other things (1) a person who does something. physicist or (2) a follower of a school, socialist, etc. I felt the common Romance form, -ista, came in from Italian, barista, and means the person who makes espresso and similar coffee drinks. (The bar in barista is from the English bar. It's from a Greek suffix for forming agentive nouns, cf. antagōnistēs (whence English antagonist) for 'rival' (literally 'one who struggles').

Ceci n'est pas un seing.