downing = eating The Collins dictionary is an excellent resource. https://www.collinsdictionary.com/dictionary/english/down_1 down 1. verb If you say that someone downs food or a drink, you mean that they eat or drink it. He has been up all night, downing coffee and typing blog posts. [VERB noun]

A smoothie is more a drink. 'downing' is slang for drinking very fast, almost without taking a breath.