An old, formal word meaning "to which place" or "to which thing" — and, in questions, "to what place? to what end?". You meet it in old writing: "the room whereto he was taken", "whereto shall I compare this?". Two things are happening in this small word. Most often it is a joining word standing for "to which": instead of "the goal to which we work", older writing says "the goal whereto we work". Less often it is a question word, "where to?" pushed into one word, and there it can slide from place to purpose — "whereto does all this lead?" is really asking what the point is. Today nobody speaks like this. Modern English splits it up and puts the little word at the end: "Where are you going to?" or simply "Where to?" — which taxi drivers in every English-speaking city still say. So the pieces survive; only the joined-up written form has gone.

往何處、到哪裡去；（做關係詞時）「到那個地方／那件事上」（古語書面用法）