The word "unforeseeably" refers to something that cannot be predicted or anticipated. When looking for synonyms, phrases like "unexpectedly," "surprisingly," or "unpredictably" can be used. Other words that might fit include "abruptly," "suddenly," or "unannounced." "Out of the blue" or "out of nowhere" can also be used to describe a situation that is unforeseeable. Other options to consider include "unanticipatedly," "astonishingly," or "shockingly." Ultimately, the best synonym will depend on the context and tone of the sentence, as well as the level of surprise or unpredictability being conveyed.