in flagrante delicto

 
Pronunciation: /ɪn fləˌgranteɪ dɪˈlɪktəʊ, fləˌgranti/
(also informal in flagrante)

adverb

  • in the very act of wrongdoing, especially in an act of sexual misconduct: he had been caught in flagrante with the wife of the Association’s Treasurer

Origin:

Latin, 'in the heat of the crime' (literally 'in blazing crime')