In paintball, the act of a paintball field allowing it's paintballers to bring their own paintballs instead of having to buy the field's paintballs.
Guy2: No, it's byop!
"Bring your own pen"
An acronym used in Geocaching, indicating that part of the cache in subject involves a paper log to sign. At times, this cache may contain only the log - bring something to write with if you hope to prove to others you found it, and don't be disappointed.
Caitlin: Right. If I only brought a pen with me, I could have been the last person to sign that cache. Darn it...