Meaning 'Boyfriend-over-Shoulder' :
1.When you are texting someone and your boyfriend is being cheeky so you tell them 'BOS' to warn them, usually signifying that you are talking about something you don'twant your boyfriend to see, or he's doing something to distract you (kissing you everywhere; doing anything he can to lure your attention away) from conversing.
2.Can also mean when a boyfriend is completely nonchalant about public displays of affection (usually right in front of people; mostly a term for posessive/insistent boyfriends because it's their way of showing affection, trying to gain more of your attention and constantly let you know they are there).
3.When someone's in a mood and you're trying to do important stuff, but there he is at your neck and ear, wandering hands- The works. Basically urging you to reciprocate.
'Sorry, can't talk.'
While we were talking to her, he had a huge case of BOS the whole entire time- It was really adorable.
Well, there goes any chance of trying to finish my assignments with him BOSing like that.