Katt Williams -- Here's Why I Fought That 17-Year-Old (VIDEO)

Katt Williams has a rambling explanation for his fight with a 17-year-old ... he thought he was 19.

Katt actually goes on, and on, and on, telling his audience Friday night at Barclays Center in Brooklyn how he got into the scuffle.

In the end, he wants to make one thing clear ... he didn't get his ass whipped and pity the poor fool who dares to try.

BY THE WAY ... IT'S SUPER PROFANE, so you've been warned.