I read this somewhere, too but if memory serves, the term was a nickname in reference to the crane grab arms that held Grandslam 22,000 "Earthquake" bombs into the belly of the Lancasters specially modified to carry it  (BI Specials with an extra powerful Merlin series engine).  I belive that on release they had similar distinctive outline to Mickey Mouses's ears.  You might be able to locate a picture of some Lancs that were outfitted with these.  Not positive but I think 617 and maybe No. 9 Squadron were pretty much the exclusive droppers of Grand Slams (operationally at least) on some railway viaducts, bridges and submarine pens towards the end of the War though I'm sure the BDU did extensive testing and maybe some other S.D. Squadrons as well.

It was, as far as I know, the heaviest conventional explosive bomb carried by any aircraft though I believe the USAAF modified a B-29 Superfortress to carry 2 of them underwing in test drops shortly after VE day.  The Lancaster was the only ETO aircraft capable of hoisting the Grandslam and even it struggled to obtain the designed drop altitude for optimal effectiveness.

Hopefully someone can confirm or refute the term as it related to the Lanc. Hope this helps.