Q: What is forbidden in the eighth commandment?
A: The eighth commandment forbiddeth whatsoever doth or may unjustly hinder our own or our neighbour's wealth or outward estate.

1 Timothy 5:8. But if any provide not for his own, and specially for those of his own house, he hath denied the faith, and is worse than an infidel.
Ephesians 4:28. Let him that stole steal no more: but rather let him labour, working with his hands the thing which is good, that he may have to give to him that needeth.
Proverbs 21:6. The getting of treasures by a lying tongue is a vanity tossed to and fro of them that seek death. 
2 Thessalonians 3:7-10. For yourselves know how ye ought to follow us: for we behaved not ourselves disorderly among you; neither did we eat any man's bread for nought; but wrought with labour and travail night and day, that we might not be chargeable to any of you: not because we have not power, but to make ourselves an ensample unto you to follow us. For even when we were with you, this we commanded you, that if any would not work, neither should he eat.

Westminster Project - Thomas Watson provides further understanding for the eighth commandment