Posting up (sitting up and taking both hands off of the handlebars) like an asshole at the end of a cycling race or a group ride when you won the sprint in for 16th...or anything but for the win.
The Cat 5 Salute (Category 5 is for beginners) demonstrates you are a complete noob at riding/racing your bike.
Often times accompanied by losing control and crashing.
Popularized on YouTube by NorCalCycling
"Did you see that guy do the Cat 5 Salute at the end of the group ride?" "I don't see a guy...oh wait, did he hit that speed bump and crash over that guardrail?" "Yup..."