if things go poorly; at worst; if one is careless
Used to introduce a possible negative outcome or worst-case scenario. Often implies that a small mistake or bad luck could lead to a serious result.
If things go poorly, this plan might fail.
At worst, I could have been seriously injured.
Derived from 下手 (へた, 'unskillful') + する (to do) + conditional と, literally 'if one does poorly'. The phrase grammaticalized into a set expression meaning 'if things go wrong'.