пускаться во все тяжкие (puskat'sya vo vse tyazhkiye)

Literally: To set oneself into all the grave things

Example: “После развода он пустился во все тяжкие.” — After the divorce he went off the rails.

English equivalent: To go off the rails

Origin: From church bell-ringing, as Russian Wiktionary explains: the heavy bells of the bell tower were called тяжкие, and ударить во все тяжкие was to ring all of them at once, which the church rules allowed only on special occasions. From that full peal came the sense of throwing off every restraint, and now of plunging into a life of excess and vice.