Slothful - Indolent, lazy, who had done nothing. God will judge people not merely for doing wrong, but for not "doing" right. See Matthew 25:45. That servant was "wicked," because he had such an opinion of his master; he had shown that he was slothful by not making good use of the talent, Matthew 25:27.
Thou knewest - This should be understood, and might have been translated, as a question. If you knew that I was Such a man you ought to have acted accordingly, so as to have escaped punishment. Didst thou know that I reap, etc.? Then thou shouldst have given my money to the exchangers, etc. This is not intended to "admit" that he was such a man, but to convict the slothful servant of guilt and folly in not having been prepared to meet him.

His Lord answered and said unto him,.... Resenting, as he well might, not only his indolence and sloth, his neglect of his duty, and his worldly disposition, but the abusive character he had given of him, in order to cover his own wickedness:
thou wicked and slothful servant; a very just character of him: he was a "wicked" servant; all men in a state of nature are wicked; they lie in wickedness, and are under the power of the wicked one; and there are many wicked men among professors of religion, and many wicked ministers, who, though not openly profane, yet either trusting to their works, or doing the work of the Lord deceitfully, or wholly neglecting it, justly merit this character. This man's wickedness lay in his slothfulness, in not doing the good he might, and had gifts and abilities for; and in entertaining wrong thoughts of, and in bringing false charges against his Lord: and he might be truly said to be "slothful"; since he took no pains to improve in spiritual knowledge; and instead of digging for that, as for silver and hid treasure, dug in the earth, and hid his talent there: he neglected the gift that was in him; did not stir it up, or study to show himself a workman that needeth not to be ashamed; did not give himself up to reading, meditation, and prayer; but trusted to, and depended on what other men had done; stole away his neighbour's words, reaped that for which he had not laboured, and entered into the labours of others; and being afraid of difficulties, indulged himself in ease and pleasure, served his own belly, and not the Lord Jesus; he gratified his worldly lusts, and had no regard to his master's interest,
Thou knowest that I reap where I sowed not, and gather where I have not strawed: not granting that he was such an one, and that his servant knew him to be such, and had given a true character of him; but supposing he was such a person he had wickedly represented him to be; he turns the argument upon him, that therefore he must needs know, that he expected to have had his money improved, and to have received it with an increase; and that upon such a consideration he ought to have been the more diligent and industrious, in using and improving his talent, and not to have indulged sloth, and idleness; and thus he convicts, judges, and condemns him, as a wicked, slothful servant, by his own words.

Thou wicked and slothful servant. The wickedness of the servant is demonstrated, not only his unfaithfulness, but by his false and slanderous excuses.

Thou knewest - That I require impossibilities! This is not an allowing, but a strong denial of the charge.