imponderable (adj.)

1794, "weightless," from assimilated form of in- (1) "not, opposite of" + ponderable (see ponder). Figurative use, "unthinkable," from 1814. As a noun from 1829, originally meaning heat, light, electricity, etc., as having no weight. Related: Imponderably; imponderability. Imponderous is attested from 1640s as "without weight." Imponderabilia "unthinkable things collectively" is attested from 1835.