I'm trying to learn more about idioms and expressions, and I came across this (which is apparently said when making a [pinky] promise):
Casting the first part aside, which is technically already a 四字熟語 for pinky promise; the second part of "針千本呑ます" has gotten me confused. Looking up the definition of the phrase on Weblio returns "swallow a thousand needles" for that section, however when I searched Jisho for "針千本" specifically, the word was defined as "porcupinefish".
I understand that 呑む signifies an action of precisely drinking or swallowing, in which case the needles would make more sense; but I can't see how the verb 呑む can't be used in this sense in a somewhat symbolic manner — not exactly having to "drink" the creature.
Which definition is the phrase exactly pointing towards? And more importantly, is there a reason behind it? Or is it just an unexpected coincidence? Although neither ingesting needles nor eating porcupinefish would be an enjoyable experience.