gitlabhq/doc/workflow/wip_merge_requests.md

"Work In Progress" Merge Requests

To prevent merge requests from accidentally being accepted before they're completely ready, GitLab blocks the "Accept" button for merge requests that have been marked a Work In Progress.

To mark a merge request a Work In Progress, simply start its title with [WIP] or WIP:.

To allow a Work In Progress merge request to be accepted again when it's ready, simply remove the WIP prefix.