Here is the sentence I came across the word 「でがす」.
とりあえず これはアッシからのおみやげでがす。 受け取っておくんなせえ。
I'm not sure if it is an archaic form of the copula「です」. Is there any nuance between them?
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Sign up to join this communityIt is certainly a form of です, but it is a dialect rather than an archaism. According to this, using でがす is seen in Kanto region and up, but sporadically.
Apart from real dialect, でがす is used to indicate unrefined character, which may be the case with the sentence in the question. A typical character is an untidy guy, maybe with beard, who is a 子分 type -- I'm not sure of what should be an appropriate translation for 子分, maybe a henchman?. See also this.