Brewer's: High Jinks
He is at high jinks. The present use of the phrase expresses the idea of uproarious fun and jollity.
“The frolicsome company had begun to practise the ancient and now forgotten pastime of High Jinks. The game was played in several different ways. Most frequently the dice were thrown by the company, and those upon whom the lot fell were obliged to assume and maintain for a time a certain fictitious character, or to repeat a certain number of fescennine verses in a particular order. If they departed from the characters assigned ... they incurred forfeits, which were compounded for by swallowing an additional bumper.” —