Nirukta is one of six Vedangas which are very essential to completely understand Vedas. Nirukta is the the study of origin of Sanskrit words i.e etymology which is very useful for understanding and interpreting Sanskrit words.
Now, when I was reading preface of Rigveda by Gayatri Pariwar, I found the following line (you can read it online at Awgp Literature:
The English translation (meaning) of the line underlined is:
Even Niruktakara Yaska mentioned/counted around 400 words of Vedas of which meaning is not known to him
I want to know which are those 400 words of which meaning is not known/understood to even Niruktakara Yaskacharya? (Is there a list of those 400 words available?)