Nondescript Beverage is a beverage that Tim drinks occasionally.
Nondescript Beverage originally appeared in Episode 10 of Super Mario Maker, Finally a CHALLENGE! at 2:20. The running joke is that Tim keeps drinking it, and despite his terrible camerawork (He once commented that his fingers had "never been on so much film"), the drink never goes on frame.
Tim confirmed that it was, in fact, a beverage, but no further details were given. JT created the hashtag #StillNotAnAd for videos in which the beverage is mentioned.