inkhorn. late 14c., "small portable vessel (originally made of horn) for holding ink," from > ink (n.) + horn (n.). Used attributively as an adjective for things (especially vocabulary) supposed to be beloved by scribblers and bookworms (1540s). An Old English word for the thing was blæchorn.

Supposably is quite rare in actual use; I believe it's more an inkhorn term than one actually found in the wild, so to speak. Thus, I fear that when you encounter it you are probably really encountering somebody who meant to say "supposedly".

Apr 20, 2014 · However, readers who found aureate terms pretentious began to call them inkhorn and inkpot terms, both references to the receptacles scholars carried to hold ink. 2002, Simon Horobin; Jeremy J. Smith, An Introduction to Middle English, Oxford: Oxford University Press, →ISBN, page 73:

Mar 6, 2013 · Etymology of "inkhorn" 4. etymology of the phrase "at all" 11. What is the etymology of 'physician'? 7.

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