Joey debated with himself as to whether or not 'supposably' was really a word: "Supposably. Supposably. Did they go to the zoo? Supposably."
I'm glad I keep up with this thread, I've now taken on board "supposable" and "supposably" .. sounds much better than the supposedly which I've used up till now.
(BTW, MY pet peeve is folks who render TILL, used as in my previous sentence, as 'TIL..)
I'd never ever heard supposable & supposably, but the OED has a long entry on the two, with many instances of supposable, going back to 1643.
For supposably, they have this;
Hence
supposably adv. (chiefly U.S.), as may be supposed; presumably.
a
1866 J. GROTE Exam. Util. Philos. (1870) vi. 107 The happiness of any supposably actual being.
1881 RUSKIN Love's Meinie I. iii. 134 This aesthetic water~hen..lived at Cheadle,..in the rectory moat,..‘always how~ever leaving it in the spring,’ (for Scotland, supposably?).
1883 Science I. 94 Conditions affecting two celestial objects which are supposably near enough to be influenced alike.
1893 ‘MARK TWAIN’ Pudd'nhead Wilson ii, Sitting on a wheelbarrow..at work, supposably, whereas he was in fact only..taking an hour's rest.