I d have thought its euphemistic sense was self evident I ve heard it used in the UK occasionally though it s certainly an Americanism The original movie Airplane uses it this way too even going so far as to name the inflatable emergency co pilot Rock just as a set up to this joke you mean we are going to have to Blow Rock Other than this rather anecdotal evidence though I can t point to any reference that includes this usage 11 03 1 Dec 2003 UTC