The secret that Nick Fury told Odinson which made the Asgardian unable to lift Mjolnir will be revealed in March 22's The Unworthy Thor #5, according to long-time Thor writer Jason Aaron. The former God of Thunder and previous wielder of Mjolnir has been "Unworthy" since 2014's Original Sin, leaving Jane Foster to take up the mantle of Thor.
“I always said we'd eventually find out just exactly what Nick Fury whispered to Thor Odinson that made him unworthy,” Unworthy Thor writer Jason Aaron told ComicBook. “The words have been scribbled in my trusty Thor notebook for years now. Words that speak to the overall story I've been telling with this character going back to Thor: God of Thunder #1. The next phase of that story begins now.”
The Unworthy Thor has sent Odinson on a quest to claim a new Mjolnir - that of the former Ultimate Universe. The story will end with a showdown between Odinson and Thanos - and of course with the revelation of Odinson's secret.
Aaron gave no indication what the revelation would be, but editor Wil Moss did have some insight into how the secret fits into the overal tapestry of Aaron's ongoing Thor saga, saying, “I remember when Jason told me what it was that Nick Fury whispered, it just felt so satisfying - like, it’s definitely a big curveball, but it feels so organic to Jason’s overall larger Thor story - it feels earned.
"If you’re looking for some insight into who Thor Odinson is as a character and if you’re looking for a sense of where Jason is going to take things from here (both with Odinson and with Jane – and with all Ten Realms!), then brace yourself, ‘cause this secret hits harder than Mjolnir.”