"No Cap" basically means "no lie" or "not lying". It is used when you want to tell someone
you're not lying to them. People sometimes tend to use emojis, like "🚫🧢" (No Cap with emojis) as an
alternative.
The term "No Cap" originated from a live streaming site called Twitch.TV, where Kapp is a popular chat emoticon on an emote extension for twitch.tv called "FrankerFaceZ". Kapp originated from a popular twitch emote called Kappa. Live streamers and their chats often use "No Kapp" to indicate they aren't lying.
Used by many popular live streamers, the phrase "No Kapp" was slowly picked up by viewers as "No Cap" and it spread all around
the internet, which is why it's still sometimes used today.