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2.
1. This is when you get a random erection.

or

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!"
by SGregz September 13, 2008
 
1.
A good thing and/or congratulations
'I've just found the original copy of the bible!'
'Bob on'
by MagnumDaggerfan November 03, 2006