Ἐξίσταντο: how do we get from 'I stand' to 'I am amazed'

ἐξίσταντο δὲ καὶ ἐθαύμαζον λέγοντες Οὐχὶ ἰδοὺ πάντες οὗτοί εἰσιν οἱ λαλοῦντες Γαλιλαῖοι;

they were all amazed and wondered, asking, "All of these people who are speaking are Galileans, aren't they?

Acts 2:7 begins with the verb form ἐξίσταντο, which is parsed as the 3-P imperfect middle/passive indicative of ἐξίστημι.

I say 'middle/passive' because the imperfect, as part of the present system, has identical forms in the middle and passive; but by sense, ἐξίσταντο is a middle.

Ἐξίστημι is a compound verb, formed from the prepositional prefix ἐκ- / ἐξ- ('from, out from') and the root verb ἵστημι.

Ἵστημι is a difficult verb, with a basic meanings of 'stand (something) up', and 'stand (yourself) up, be standing'.  It is used over 150 times in the New Testament, in a large variety of forms. 

Consider the following: 

(1)  καὶ ἔστησεν αὐτὸν ἐπὶ τὸ πτερύγιον τοῦ ἱεροῦ

and he set /  stood him on the pinnacle of the temple  (Matthew 4:5)

The form of ἵστημι here is ἔστησεν; an aorist active indicative.  It is transitive, meaning that the subject ('he' - the devil) performed an action ('set / stood') affecting someone or something else as a direct object ('him' - Jesus).

The direct object of a transitive verb is generally in the accusative case, as it is here: αὐτόν.

Now compare that use to this:

(2)  καὶ αὐτὸς ἦν ἑστὼς παρὰ τὴν λίμνην Γεννησαρέτ

and he (Jesus) was standing by the lake of Gennesaret   (Luke 5:1)

Here, ἑστὼς is a perfect active participle (masculine singular nominative) of ἵστημι. In the perfect this verb is intransitive and refers to a completed action with ongoing consequences; i.e., at some point Jesus stood up, and he is now standing. 

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So what about ἐξίσταντο in Acts 2:7?  The usual translations for most of the seventeen uses of this verb in the New Testament are some version of 'to be amazed, to be astonished', etc.

How does 'amazed' relate to 'stand'?

The prefix ἐξ- seems to indicate that you are standing 'outside' of yourself in some way.  English has a similar idiom: 'I am beside myself'.  Although it doesn't correspond exactly to 'I am astonished', you get the basic idea, which also explains the middle voice.