Mat 9:2 Son, be of good cheer; thy sins be forgiven thee.
Be brave, child. they are letting go of themselves, your mistakes.
This is the first mention of "sin" by Christ in the KJV of the gospels, even though the Greek words used do not actually mean either "sin" or "forgive" in Greek. More about these and related words in this article.
The verb translated as "be of good cheer" is from a noun that means courage. It is best translated as "have courage" or "be brave."
The word translated as "son" is not the usual word for example, used in "son of man" but another word that is usually translated as "child."
The word translated as "sin" is a form of a word that means "to fail in one's purpose", "to neglect," and "to be deprived of." It has no sense of doing malicious evil in Greek. The best English translation is "mistakes" or "failures" rather than what we commonly think of as the evils of "sin." More about this word in this article.
The word translated as "be forgiven" primarily means "to let go" or "to send away." This same word is translated as "leave", "forgive", "suffer," and "let" in the New Testament. With the word translated as "sin" it is translated as "forgive" even though it doesn't really mean that in Greek. Its form is that in which the subject is acted upon by itself. It is in the present tense so "are being let go by themselves".
There is no Greek word "thou" in today's sources.
Mistakes leave of their own accord.
ἀφίενταί (3rd pl pres ind mp) "Be forgiven" is from aphiemi, which means "to let fall", "to send away", "give up", "hand over", "to let loose", "to get rid of", "to leave alone", "to pass by", "to permit," and "to send forth from oneself."