Jeremiah 18:12King James Version (KJV)
12 And they said, There is no hope: but we will walk after our own
devices, and we will every one do the imagination of his evil heart.
My understanding after a quick reading is that Jeremiah is unsuccessful in getting the men of Judah to believe he is a messenger from God and to take him seriously.
He leaves them to their "own devices" and asks God to deal with it.
Devices are plots and schemes. For example they plot to kill Jeremiah.
- a plan, method, or trick with a particular aim.
"writing a letter to a newspaper is a traditional device for signalling dissent"
synonyms: ploy, plan, cunning plan, tactic, move, means, stratagem,
scheme, plot, trick, ruse, gambit, manoeuvre, machination, intrigue,
contrivance, expedient, dodge, artifice, subterfuge, game, wile;
Bible scholars will be able to give a much more accurate reading than I have.