also clock-work, 1660s, "mechanism of a clock," from clock (n.1) + work (n.). Figurative sense of "anything of unvarying regularity" is recorded earlier (1620s).
Also, regular as clockwork. With extreme regularity, as in Ruth arrives every Wednesday morning just like clockwork, or You can count on his schedule, which is regular as clockwork, or Their assembly line runs like clockwork. This idiom alludes to the mechanical and therefore very regular action of a clock. [Second half of 1600s]
