Impassively rhymes with:
passively, massively, passively, aggressively, cohesively, comprehensively, compulsively, conclusively, decisively, defensively, derisively, effusively, excessively, exclusively, expensively, explosively, extensively, impressively, impulsively, inconclusively, inexpensively, intensively, massively, obsessively, offensively, passively, persuasively, progressively, reflexively, successively
Impassively sounds like:
impassable, impeachable, impeccable, impeccably, impossible, impossibly, infeasible, invisible