Exactly: επ [ép] επιφ. : συνήθ. δηλώνει ξάφνιασμα, ξαφνική κίνηση κτλ.: επ, σε τσάκωσα! [ηχομιμ.]
Btw, there's a joke about the allegedly shortest dialogue in Greek, when someone opens the door of an occupied bathroom:
—Επ! (the occupant, surprised and exclaiming that the bathroom is occupied)...