Diamond hands is framed as conviction. Sometimes it is; often it is sunk-cost-fallacy with a hashtag. There is no way to distinguish the two except by whether the holder had a reasoned plan for the position.
Its counterpart is paper-hands. See also hodl.
Example: holders who kept a meme stock from $300 to $40 were praised for diamond hands. The stock did not return to $300.
Related: paper-hands, hodl, bagholder, sunk-cost-fallacy