I know WHAT it is, but I dont know why it exists, what use it has, or where it may have come from. All sides are identical, its translucent green. The pips are scalloped, not painted on, and are otherwise unremarkable. Help me out here, its definitely my favorite d6, but I am struggling to figure out a purpose for a +5 dice.
...I mean, aside from that I need a +5 weapon, clearly.
You come off like an ass, but I'll give the benefit of the doubt that I'm reading that into your words and will answer assuming as much.
A trick coin has the benefit of being deceptive. You can only see one side of a coin at a time. This however only shows 5s. It clearly only shows 5s to the point where from basically any angle, it shows not one, but three 5s. As the other commenters suggest, this does appear to be a forcing die, but also as suggested, its probably for really crappy magic tricks, and won't actually work as a trick die.
A trick die, like a trick coin, is intended to pass scrutiny in use. An all 5 dice may make it a round in yahtzee but its green, so this one at least is obviously different fom most die. If it is actually supposed to pass scrutiny, I'd go with a weighted die.
As for what it could possibly be, cheapo brand magic tricks wasn't something that crossed my mind, but fits. I gave how I would use it in my post and if I already knew what it was, I wouldn't be asking "what's this thing?" In /C/whatisthisthing.