It does not come from the shape of a rope. It comes from the shape of air stream. Originally it means the hardness of the word. I check it from the book<shuo wen jie zi>, which is the earliest dictionary edited by Xu Shen in 122 A.D.

The original meaning of the character is "again", "once more", "repeating". Hence the pictogram of a curly rope or string. As a pronoun, "you included", "yours", "his" etc. As a verb, "is", "are", etc As adverb, "just then", "but", "hence" And can also be used as a conjunction

you can check at the website,it is very clear:http://qiyuan.chaziwang.com/etymology-17655.html

Well, before posting a question here I always try to find the answer myself and that website is the first I check. In this case didn't help much. The etymology of this character doesn't seem to be so clear, that's why I asked. Some say comes from the shape of a rope, other that symbolizes a pregnant woman... Thanks anyway.

Wiktionary usually helps here: https://en.wiktionary.org/wiki/乃