The other two definitions are correct, but omit the origin of the term. It was originally "mini-mittreilleuse" (not sure of spelling on that), a French neologism meaning "small machinegun". It's a 5.56mm weapon that fires the same NATO standard rifle ammo as the M16. It is normally fed from 200-rd belts, but can also accept 30-rd M16 magazines.
The Minimi is not a popular weapon because it is prone to malfunctions and the 5.56mm projectiles don't penetrate much in an urban environment.
In Hawaii, a local store (usually a Mom & Pop operation) that sells soda, candy, peanuts, sunflower seeds, etc. The sunflower seeds are the origin of the name. You crack the sunflower seeds open to eat them, so they are called "crack seeds".
Hey bruddah, you want go crack seed store for get a snack?