supposititious

Spurious; substituted for the genuine, counterfeit; fake.

[...] the said pretended Testament was supposititious, & contriued by such as meant to defraud both the heires female of the said king Henrie the 8. as well as these of his eldest sister [...]

But it may bee some will here obiect and say; that the Haire, and Loue-lockes which they weare, are supposititious, false, and counterfeit, and not their owne: therefore they violate no Law of God, nor Nature, since the long Haire they vse, is but borrowed, and aduenticious, their owne being short enough: perchance, but little or none at all.

Imaginary; fictitious, pretended to exist.

His good sense had pointed out to him the artifices of the monks, and the gross absurdity of their miracles, wonders, and supposititious reliques.

1836, Edgar Allan Poe, Review of Joseph Rodman Drake, The Culprit Fay, and other Poems and Fitz-Greene Halleck, Alnwick Castle, with other Poems in Southern Literary Messenger, Volume 2, No. 5, April 1836, pp. 327-328, … we discover in all men a disposition to look with reverence upon superiority, whether real or supposititious.

Supposed or hypothetical.

You might hope to get some other nonsensical belief into the head of George Gradgrind, or Augustus Gradgrind, or John Gradgrind, or Joseph Gradgrind (all supposititious, non-existent persons), but into the head of Thomas Gradgrind—no, sir!

Far more extravagant (insanior) yet is the idea of the whirling of the supposititious primum mobile, which is still higher, deeper, more immeasurable; and yet this incomprehensible primum mobile would have to be of matter, of enormous altitude, and far surpassing all the creation below in mass, for else it could not make the whole universe down to the earth revolve from east to west, and we should have to accept a universal force, an unending despotism, in the governance of the stars, and a hateful tyranny.