When the trousers/shorts/leggings of a person ride up into ones crack, resulting in a clearly visible outline of the persons butt crack. This can usually be seen on buses or other forms of public transport.
It origionates from the word 'cameltoe', but is the 'back' variation because it isn't asociated with the 'front' (vagina).
'I saw an epic bameltoe on the bus the other day, you could proper see the guy's butt crack!'
'Thats awesome, you should have taken a photo!'