"Embrace the grump" is a phrase that encourages people to accept and even celebrate their grumpiness. It's a playful way of acknowledging that everyone has bad days or moments where they're feeling a bit irritable or out of sorts, and that's okay. Instead of trying to hide or suppress those feelings, "Embrace the grump" suggests that we should own our grumpiness and not be ashamed of it.
When used as a t-shirt title, "Embrace the grump" could be seen as a humorous way of warning others that the wearer might not be in the best mood. Alternatively, it could be worn as a badge of pride by those who identify with being grumpy or cantankerous. Ultimately, the message behind "Embrace the grump" is one of self-acceptance and embracing all parts of ourselves, even the less pleasant ones.