Beware of that wolf in sheep’s skin the closest meaning to this idiom is

The idiom it’s all Greek and Latin to me means

The thief was caught . The suitable idiom is

When the police interrogated the thief, he made a clean breast of everything.(Meaning of the idiom underlined)

Meaning of the Phrase 'Ad hoc'. 

Replace the underlined phrase with one word: '' I did not expect such childish talk from a grow-up man".