English is an idiomatic language - peppered with phrases and Figures of Speech. Some people criticize the English language for being illogical. Indeed, idioms provide a good example for their argument. But I believe that the strength and beauty of English is rooted in the fact that it is illogical. Language is simply a means to an end - not an end in itself.
Language is simply a means of expressing the human mind.
The mind is illogical. So this aspect is obviously reflected in the language we speak. Idioms add colour and rich imagery to the spoken words. Shiver is used in the sense of 'shatter' - splinter into pieces. A promise to pay off an outstanding debt.