Tartaróō (ταρταρόω) means to cast into Tartarus — the lowest abyss beneath Hades. In Greek mythology, Tartarus was the prison for the worst offenders. Peter borrows this term for divine judgment on fallen angels.
Second Peter 2:4: 'God did not spare angels when they sinned, but cast them into hell (tartarōsas), putting them in chains of darkness to be held for judgment.' This is one of three examples of God's certain judgment (the others: Noah's flood and Sodom). If God did not spare even angels, how much more certain is judgment on unrepentant humans? Yet Peter also affirms 'the Lord knows how to rescue the godly from trials' (v.9). Divine wrath and divine rescue go together.