When a person drinking a 40oz. malt liquor consumes the drink down the top of the label. After getting to the top of the label, orange juice is poured into the bottle, filling it back up. It's usually done with a lower alcohol malt beverage, such as a 'Hurricane' although it can be done with higher alcohol 40's like 'Steel Reserve: High Gravity"
It derives its name from its more popular relative, the mimosa, drink of Champagne and Orange Juice.
"That niggas rockin' a thug mimosa, respect!"