I have asked this question to a few of my Rabbeim and none have provided me with an answer.
How can something that isn't an issur d'oraysa or an issur d'rabanan, become assur?
It is extremely common in Jewish Law, that when a minhag becomes widely accepted, it has the full force of "being assur". I would like to know exactly how something that neither Hashem nor any Sanhedrin prohibited, can now become assur?
In order to help answer my question, here are a few examples that the Mishna Berura or Shulchan Aruch says are assur, that started as minhagim:
- Fasting (besides Yom Kippur and Tisha B'AV) during non-Shmad times.
- Most of the three weeks/ 9 days halachos.
- Not passing things to your wife when she is a niddah.
There are many more but I want to stay on topic. The bottom line is: How can a minhag become codified into halacha?