erstwhile looks to me like a half German, half English word.

It's all-English, Brannie. Old English ærest 'first, at first, before all' (link) and German erst 'first' are related. Interestingly enough is the -st ending which signifies the superlative degree of an adjective. I think German ehemalig 'erstwhile, former' would work better in this case. Mal, related to English meal, means time (as in time to eat): e.g., zwei mal zwei is 'two times two'. .

Ceci n'est pas un seing.