This article contains spoilers from the most recent DCEU film. You have been warned.
Harley's Mallet is a mallet used by Harley Quinn. It appears to be an ordinary wooden mallet. However, she painted a smiley on it.
Preparation for the Battle of Midway City
The mallet is among the items, clothes, and weapons returned to Harley. She later opted to use her bat instead and left the mallet behind.