Based on a Naaleh.com shiur by Rabbi Beinush Ginsburg
Avak lashon hara is not actual lashon hara but involves anything that is associated with lashon hara and that can lead to it. While lashon hara is a Torah prohibition, avak lashon hara is a Rabbinic prohibition. The first category of avak lashon hara would be insinuating something negative. “Who would’ve thought Shimon would turn out the way he did,” or “I don’t want to speak any lashon hara about Reuven,” are examples of avak lashon hara where nothing negative is actually said but there is a veiled hint.
The second category of avak lashon hara is praising someone excessively in public. The Gemara writes, “Al yisaper shivcho shel chavero…”- A person should not praise his friend for he will end up discussing his faults as well. This does not mean that one should refrain from praise completely as we see many instances in Chazal where people were praised. Rather according to Rashi this means that one should not praise excessively and according to the Rambam it means that one should not praise a person in front of his enemies. This includes praising someone in public as there is bound to be someone who will say something negative. The one exception is a great tzaddik who may be praised publicly as even if something evil is mentioned, everyone will dismiss it as false.
In light of this halacha, how do we understand the custom of excessively praising a chassan and a kallah or a bar mitzvah in public? Normally, at a simcha, people expect the chassan and kallah to be praised. Therefore there is no concern that people will get excited about excessive praise or that it will lead to negative comments. Similarly, the Maharsha notes that one is allowed to praise ones Rebbe because every student knows to praise his Rebbe so it will not lead to jealousy or lashon hara.
May we merit to purify and elevate our speech and may this helps us attain spiritual perfection for the coming year.