In Black Clover, at least in the anime, Asta frequently shortens します (shimasu) into しゃす (shasu). Is that supposed to be indicative of an accent or dialect, or is there some other reason for him to do that?
In short, he is speaking colloquially in slang, not ant dialect in particular.
For an example:
オナシャス (onashyasu) means お願いします (onegaishimasu, lit. please)。 「お願いします (onegaishimasu)」
This is like shortening "please" to "pls" or "plx".
This speech pattern is suppose to outline how crudely (or uncouthly) Asta speaks, in contrast to the rest of the other Knights.
Asta's manner of speaking isn't dialect and moreso shortened impolite slang rather than an actual dialect change, which would be visible with differences in vocabulary and verbage.
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