Something or someone who goes up behind you, and startles you, whether in a sexual manner, or in a non-sexual manner.
John and james are playing call of duty.
John: Oh man, where are those noobs?
John gets killed.
James:Yo john, where are they at?
John: They're on your rondel! (meaning they are behind him)
Jack is talking to his friend, Jim about his time last night with his girlfriend, Jill.
Jack: So, last night, Jill came over to mine.
Jim: Hows it going with you two?
Jack: Well, lets just say she may never need to worry about someone taking her up the rondel again.
Jim: (gives a disgusted look)