Think how uncomfortable you would feel and look if you were sitting on a chair after someone had put biscuit crumbs in your underpants. Thats where the term comes (or crumbs?) from.
I have shamed myself with that pun and i apologise unreservedly to my family, my friends and my future children who will now have to live with the shame, the ridicule, my thoughtless punning has wrought upon (a pun?) them. Oh no. No, not again. Please lord, HELP ME STOP THIS MADNESS!!!