formidable; fearsomely impressive; intimidating
They are redoubtable warriors, capable of defeating armies several times larger than their own.
redoubtable can be re+doubt (able). so if you have REpeated DOUBTs in any subject just before the exam, it might CAUSE FEAR Someone who is ABLE to READ your DOUBTS without you even expressing them is awe-inspiring and worthy of honor. it can be splitted like re+doubt+able considering which can show that the thing which is able to create doubt again and again. and those things always create fear.