不得其解百思不得其解bù dé qí jiě

be at a loss as to the meaning/“𧹞郎”， 旧时注家～， 其实就是银矿或铜矿的冶炼工人。(郭沫若《李白与杜甫》189)Ancient annotators were at a loss as to the meaning of the phrase “red men”. The faces of the metal workers labouring by the furnace were scorched red,so much so that they intensified the brilliance of the moon.Hence “red men. ”