So ordinary and usual that it's not interesting or special anymore; happening all the time.
Originally from the concept of a 'commonplace book'—a personal notebook where people wrote down memorable or useful passages. Over time, the term shifted to describe something that appeared everywhere and was therefore unremarkable.
Renaissance scholars kept 'commonplace books' to record brilliant ideas, but eventually anything found in lots of these books became seen as boring—so the word flipped from valuable to ordinary!
Complete word intelligence in one call. Free tier — 50 lookups/day.