Cat 6

An unofficial, non-sanctioned, underwhelming bike race instigated and sustained by childhood loss, common perniciousness, or bike lust.

Engaging in a Cat 6 race is done by amateurs with competitive spirits in non-racing situations like commuting or casual riding.

Reference
Comes from USA Cycling racing levels which are called categories or "cats" for short. Categories for road racing range from 1 (most skilled) to 5 (least skilled). A Cat 6 race would be amateur and therefore worse than the least skilled official Cat 5, hence the disparaging name.

Forum Introduction

 * Some Great Cat 6 Racing This Morning http://bikearlingtonforum.com/showthread.php?p=6509#post6509

Links

 * USA Cycling Racing Levels https://en.wikipedia.org/wiki/USA_Cycling#Road
 * "Cat 6: Competitive Commuting Turns Bike Rides into Races" - Good https://www.good.is/articles/cat-6-competitive-commuting-turns-bike-rides-into-races