The word "不得了" is formed through a straightforward intensifying construction: it begins with the negation "不" (not), combines with the potential complement "得" (which can indicate possibility or achievement), and ends with "了" which here carries its classical meaning of 'to finish' or 'to conclude.' Literally, it reads as "cannot be finished/concluded", creating an image of a situation so extreme or an emotion so intense that it cannot be contained, settled, or brought to an end, thus naturally evolving into its modern meaning of "extremely" or "terribly serious".