A smaller, less predictable stream which deviates from the primary urine stream. One can not predict when a substream might emerge so it is pertinent to remain alert during urination.
John had one too many beers at the rager, he knew he had to pour the lemonade, and soon. He rushed to the bathroom he tore his pants down in frantic panic. Suddenly he was releaved, but it didn’t last long as a substream suddenly emerged! As he peed, the substream spelled yelled w drained defeat as he was not prepared and could not stop it from spraying the away from his intended target.
by Future Dictionary June 23, 2018
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The opposite of 'mainstream'. Applies to music (modern and older), books, hobbies, movies, etc.

The 'Is It Mainstream Music?' Test: Does this song/band play on more than three radio stations? Yes- Most likely main. No- Most likely sub.
Miley Cyrus and Jay-Z? Mainstream. Freezepop? Substream.
by Unexist May 16, 2010
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