Kommentar zu Bereschit 24:21

משתאה WAS WONDERING — The word really has the idea of being waste and desolate, as (Isaiah 6:11) “Until cities be waste (שאו) and the land become utterly desolate תשאה.

והאיש משתאה, the word is derived from Isaiah 6,11 תשאה שממה, and the letter ת is similar to the letter ת in the word ישתבח which is derived from שבח, or the ת in the word וישתמר in the phrase וישתמר חוקות עמדי, which is derived from the root שמר, (compare Micah 6,16). Here the word משתאה means the same as משתומם, being amazed at this girl volunteering that she would also draw water for his camels. From these words of her he understood that G’d had heard his prayer. Commentators who understand the word משתאה as being related to שתייה, drink, are in error. Such an interpretation is impossible, seeing that the letter א in that word cannot be related to שתייה at all. Furthermore, Rivkah had not mentioned anything about watering the camels until after he himself had finished drinking already. At that point Eliezer began to wonder.

Wondering at her. He was amazed at her zeal.