BT Zev 18a(17) : אי מהתם הוה אמינא עבודה דמעכבא כפרה, אבל עבודה דלא מעכבא כפרה אימא לא‏ : if it were (derived) from there I would have said it (refers to) and act that invalidates atonement but I will say that it is not an act that invalidates atonement
See the entire entry for ˁkb vb. to hold back, to hesitate