git merge branchA branchB, branchB generally defaults to the current branch, so

git merge origin master //将origin merge 到 master 上
git merge origin/master //将origin上的master分支 merge 到当前 branch 上 

Generally when performing a merge operation, it is best to first checkout to the branch you want to perform the merge operation, that is, branchB, and then proceed

git merge branchA //默认为当前branch，即branchB

Because you cannot ensure whether a confit is generated, checkout to branchB first.
In addition, to be on the safe side, you can create a backup branch before merging

git checkout -b branchB_backup

This way, even if something uncontrollable happens after you merge, you can still check back.