An energic-nun (also called a “paragogic nun” or a “euphonic nun”) is a nun which is used in some verbs when a pronominal suffix is added. It has no effect on the meaning. See more in Joüon page §60f or this video.
You can see it in this verb (from Genesis 13:15): אֶתְּנֶנָּה
Karl Randolph tells me the second nun is part of the third person feminine suffix and is not a energic/paragogic nun. It should read “I give it to you and to your seed for an undetermined time” often mistranslated as forever. Hebrew does not have a word for eternity.
Thank you for the comment. The 3fs suffix in use here does not usually have a nun. It is just the qamats with the he. So the nun slips in there for who knows what reason. Again, you can see this discussed in various grammars. Which grammars do you have access to? I can point you to the page number. Davidson mentions it here:
https://www.google.com/books/edition/Davidson_s_Introductory_Hebrew_Grammar/-xMRSEYqfawC?hl=en&gbpv=1&dq=%22Singular%20suffixes%20added%20to%20Imperfect%20and%20Imperative%22&pg=PA148&printsec=frontcover