Questions & Answers about Er hat mich heimlich hintergangen.
How does the prefix hinter- change the meaning of gehen to mean 'deceive'?
The prefix hinter- often gives a verb a figurative meaning related to doing something behind someone's back. While gehen means 'to go', hintergehen translates literally to 'to go behind', which German uses to mean 'to deceive' or 'to betray'.
Why is the past participle hintergangen instead of hintergegangen?
When hinter- is used with this figurative meaning of deception, it acts as an inseparable prefix. Because inseparable verbs do not take the -ge- prefix in the perfect tense, the past participle is simply hintergangen.
Why use mich (accusative) instead of mir (dative) with a verb that implies going 'behind' someone?
You might expect the dative if you think of the spatial preposition hinter (behind), but as a complete verb, hintergehen is transitive. The person being deceived is the direct object of the action, so it requires the accusative case (mich).
Is heimlich strictly necessary here, since hintergehen already implies doing something in secret?
It is slightly redundant, much like saying 'he deceived me behind my back' in English. However, adding heimlich (secretly) is a very common stylistic choice in German to heavily emphasize the sneaky, covert nature of the betrayal.