no wonder; as expected; of course
Expresses that something is only natural or to be expected given the circumstances. Often used in response to new information that confirms a prior assumption.
He practices every day, so it's no wonder he's good.
That shop has bad service, so it's only natural that customers don't come.
Kanji spelling using 筈; rarely used in modern writing.
Rare kanji spelling with 其の; it is rarely used.
Combination of その (that) and はず (expectation, should be). The phrase literally means 'that should be the case' and developed into a fixed expression meaning 'no wonder' or 'as expected'.