In Shemot 12:9, the text reads, "אַל תֹּאכְלוּ מִמֶּנּוּ נָא " which is translated at chabad.org as "You shall not eat it rare ". The meforshim (as taken from sefaria.org) look at the word "Na" as meaning "alive" or undercooked, often based on a connection to an Arabic word. As Rashi writes, "Heb. נָא Something not roasted sufficiently is called נָא in Arabic." The Bechor Shor writes, " תרגם אונקלוס כד חי כי נא חי שאינו מבושל כל צרכו בערבי". This meaning of the word occurs only here (or, as the Ibn Ezra puts it, " ובדרך הזאת לא מצאנו בכל המקרא "). A look in the Even Shoshan confirms this.
The Rashbam writes, "I believe this means a type of frying in a pot but in its own juice (including the blood) not involving water known as צלי קדר, not roasting directly on the fire, as required by our verse here ". His practical application of halacha would be different, then.
This is intended to be a direct law applicable for the generations -- why would the text use a word which needs to be figured out, and why would the text switch to the Arabic? Was the Hebrew unable to describe "not roasted completely" or "raw"? Oftentimes, the text uses a less well known word because it imports a subtlety in meaning. What does relying on the Arabic hapax in this case add to understanding that would have been absent had the text stayed with Hebrew?