If you’re already shouting, ‘What? It’s “supposedly”, not “supposably”, you absolute twonk,’ I’m going to stop you right there. Because ‘supposably’ is indeed a word, albeit one that’s fallen out of fashion. It’s an adverb (a word that modifies or describes a verb) meaning ‘as may be supposed or imagined’ or ‘conceivably’. So it basically describes something that’s feasible.
In both sound and sense, it sounds incredibly similar to ‘supposedly’, I know. But there is a subtle difference: ‘supposedly’ means ‘according to what is generally assumed, reported or believed’. It carries a bit more doubt – if you say, ‘Emma supposedly knows what she’s talking about’, you’re basically saying I’m talking out of my arse (which may or may not be true).
With that sort of settled, here’s ‘supposably’ in action (also featuring a guest appearance from ‘supposabilities’!):
‘We have sometimes been surprised, that, connected as poetry and painting are, as sister arts, so little use should have been made of the painter’s study, or of those incidents which very supposably—as supposabilities are allowed at the theatre—may happen in the course of an artist’s life.’
—The New Monthly Magazine (London, UK), June 1829
Unsurprisingly, both ‘supposably’ and ‘supposedly’ trace their roots back to the late Middle English verb ‘suppose’, meaning, you’ve guessed it, exactly what it does now – to think, assume or imagine. Middle English grabbed that from the Old French supposer and ultimately the Latin supponere, meaning ‘to place under’ or ‘substitute’. ‘Supposedly’ is the OG, having first appeared in English around the 1610s, while ‘supposably’ arrived a bit later, around the end of the 18th century (roughly 1795).
So now you know – ‘supposably’ is a real word. Go forth and embrace the supposabilities!