"My cup runneth over" is a quotation from the Hebrew Bible (Psalms:23:5) and means "I have more than enough for my needs", though interpretations and usage vary.

Is my cup runneth over an idiom?

My cup runneth over is an idiom that has its roots in ancient times. ... English idioms can illustrate emotion more quickly than a phrase that has a literal meaning, even when the etymology or origin of the idiomatic expression is lost.

Is there a word runneth?

(archaic) Third-person singular present simple form of run.