also: もうしわけないぐらい
expression, adverb, noun which may take the genitive case particle 'no'
so ... that one feels sorry; to an apologetic degree
Used adverbially or adjectivally to express that something is so extreme (good, bad, much, etc.) that it makes the speaker feel guilty or apologetic. Often follows an adjective or verb in て-form.
彼は申し訳ないくらい親切にしてくれた。
He was so kind to me that I felt almost guilty.
申し訳ないくらい簡単に勝ってしまった。
I won so easily that I felt a bit sorry.
Phrase composed of 申し訳ない (apologetic, inexcusable) + くらい (extent). The exact historical derivation is straightforward; the expression is a fixed adverbial phrase.