1. This is when you get a random erection.
2. When joking with your mates.
(also see 'Marley')
1. "OMG i have a Bob On, i hope no one sees!"
2. "OMG i just gold stared Run to the Hills on Rock Band, i have such a Bob On!"
A good thing and/or congratulations
'I've just found the original copy of the bible!'