Make Friends You Can Call by Their Nicknames, Even When You're an Old Fart

Gintama.: Slip Arc · Episode 5 · “Make Friends You Can Call by Their Nicknames, Even When You're an Old Fart”

Consists of episodes animated from unused manga chapters, before returning to the Silver Soul Arc on January 8, 2018. Porori is an Onomatopoeia for something (mainly water) falling.