There are several synonyms that can be used in place of the phrase "no lie". The most commonly used synonym is "seriously", which indicates the speaker's sincerity and honesty. Another option is "truthfully", which also emphasizes the honesty of the speaker. The phrase "without a doubt" is another great option, indicating the speaker's certainty in what they are saying. "Honestly speaking" is another synonym that can be used to replace "no lie". Overall, the use of synonyms for "no lie" is a great way to enhance the quality of one's speech and to convey their sincerity and truthfulness.