Free from the Regulations of the Law
1Cor 6:12,13 "Everything is permissible for me"— but not everything is beneficial. "Everything is permissible for me"— but I will not be mastered by anything. "Food for the stomach and the stomach for food"— but God will destroy them both. The body is not meant for sexual immorality, but for the Lord, and the Lord for the body.
More precisely the word "permissible" is literally "lawful", used commonly in the gospels in the context of the Law of Moses, but only two other times in the New Testament letters. Consider 1Cor 10:23-25 "Everything is permissible"— but not everything is beneficial. "Everything is permissible"— but not everything is constructive. Nobody should seek his own good, but the good of others. Eat anything sold in the meat market without raising questions of conscience ..."
So by "Everything is lawful" he's not saying that sin is lawful, but rather that Christians are not under the regulations of the law, like the dietary restrictions in the law. Christians are free to derive their own applications from the law rather than being under a legalistic bondage to its regulations. Freedom in Christ does not mean we should use our freedom to sin.
"You, my brothers, were called to be free. But do not use your freedom to indulge the sinful nature; rather, serve one another in love." Gal 5:13 For "he died for all, that those who live should no longer live for themselves but for him who died for them and was raised again." 2Cor 5:15