The fighter/warlock in our party tried to make a creature drop concentration with Command, by saying "stop".
The target was not an enemy. Whether stopping concentrating would be harmful to the target is debatable, but outside of the scope for this question.
This first level spell states:
You speak a one-word command to a creature you can see within range. The target must succeed on a Wisdom saving throw or follow the command on its next turn. The spell has no effect if the target is undead, if it doesn't understand your language, or if your command is directly harmful to it.
Some typical commands and their effects follow. You might issue a command other than one described here. If you do so, the GM determines how the target behaves. If the target can't follow your command, the spell ends.
[…]
The spell suggests it could work, under GM's discretion. But I'm wondering whether the word "stop" would be clear enough, by RAW.
Additionally, if possible, what would be a better word to use to make someone drop concentration using Command?
Best Answer
The GM determines how the target behaves.
No one but the DM running your game can tell you what effect a particular command will have, because the spell states:
I can tell you what I would rule in a particular circumstance, but I have little confidence that my ruling will reliably predict any other DM's ruling, because the rules here give the DM full liberty to determine what happens, and no guidance at all for making that decision. The rules-as-written answer is that there is no rules-as-written answer. The rules themselves recuse themselves of any decision making responsibility in this instance.
That said, I would not expect "stop" to be a great choice of command in most instances, due to the ambiguity. Stop what? Stop breathing? Stop fighting? Stop right there criminal scum? When you use an off-label command, you're just rolling the dice (figuratively) on the DM's interpretation of the command, especially with one as vague as "stop".