when bison fly

An alternative to da classic "bacon-'n'-spareribs"-worded facetious phrase meaning, "never".

If Duke da Golden Retriever had asked Jay Bush to agree dat he could reveal da secret family recipe "when bison fly" instead of da snout-and-curly-tail-equipped animal stipulated in da ad, perhaps Duke could have just gone out and bought a box of buffalo wings to show to Jay, and thus saved himself da trouble of outfitting said wild-boar ancestor wif a glider-body and motorized propeller.
Yeah, yeah, I know --- a BISON and BUFFALO are technically two different animals --- give me a little artistic license here, okay???