In fact, according to the words of our Sages of Blessed Memory, in maseches Arachin (15a, 16b) and elsewhere, that deal at great length with the enormous shamefulness of this sin [of Lashon HaRa] and its great punishment (we will quote their holy words later on in the book, G-d willing), it is implicit that this sin [of Lashon HaRa] is itself very severe, regardless of that which results from [the forbidden speech]. Therefore we need to elucidate, concerning what [reasons] and why is this sin more severe than the other grave sins that are [found] in the Torah.
 Pesukim = Verses.
 MaLBI”M explains (Yishayahu: 1; 4) that עון – willful sins – relates to the words עוות and עוה – distort. עון is the type of sin one performs as a result of intentionally distorting his ways from the way of the Torah, and thereby sins intentionally. חטא – inadvertent sin – refers to a sin performed through negligence, where one thought that the action was not a sin, thinking it is permitted or being blinded to the fact it is forbidden to desire. חטא means “miss”, as in “miss the target”. Due to erroneously “thinking” that a given sin is permitted, or being blinded to its forbidden nature, the person “misses the mark”, as his understanding of the given sin is “off target”.
“Maseches” refers to a book from the Talmud.