Where did you see this used?
For slang and expressions like this, consult the "Urban Dictionary". It's not a language learning tool but it helps describe some slang and non-standard expressions. It *might* explain words and phrases that dictionaries and text books don't. It may have a heavier representation of US culture. And sometimes it won't be helpful at all, but it's another tool to explore.
https://www.urbandictionary.com/define.php?term=Let%27s%20get%20weird
Depending on the situation it could mean any of the following:
- At a party -- Let's get drunk / wasted. Let's start dancing.
- Friends hanging out -- Let's go have fun.