There was a locally-hosted Git server platform called Stash. Atlassian bought it, rebranded it as "BitBucket Server" (positioned similarly to GitHub Enterprise or self-hosted GitLab) and gradually made it look and feel like BitBucket (the cloud product), even though they're actually completely separate codebases (or at least used to be).

That explains why it doesn't suck nearly as much as Jira and Confluence...

But why was it ever called Stash? The word stash has particular meaning in git, and it's been that way since a very early git version.