Use a sharp paring knife to score the ham, then place it with the cut side facing down into the slow cooker. Pour the orange juice into the bottom of the slow cooker.
In a small bowl, mix together the brown sugar, softened butter, Dijon mustard, cinnamon, and allspice until well combined.
Spread half of the brown sugar mixture evenly over the entire ham, setting aside the remaining glaze for later use.
Put the lid on the slow cooker and set it to LOW, cooking until the internal temperature at the thickest part reads 140°F (60°C), which typically takes 4-5 hours. The exact time depends on whether the ham has a bone; a boneless ham will cook faster.
When there are 10-15 minutes left in the cooking time, brush the remaining glaze over the ham, then re-cover and continue cooking until done.
Take the ham out of the slow cooker and allow it to rest for 10 minutes. If you want a stickier glaze and a crispy exterior, you can optionally place it under the broiler for a few minutes.
Cut into slices and serve.