The phrase "most matter of course" connotes things that are expected, routine, and unremarkable. Its antonyms can be terms such as "unusual," "improbable," or "surprising." These words suggest unpredictability, uncertainty, or novelty that deviates from the norm. Other antonyms for "most matter of course" can include "irregular," "different," "atypical," and "exceptional." These words imply that the situation or occurrence is not a typical or standard one. Additionally, antonyms such as "remarkable," "noteworthy," or "extraordinary" indicate that the event stands out from the ordinary and deserves attention.