A stock phrase
is one which gets used over and over (and over and over (and over and over)) in a particular genre
For example, "sleeping with the fishes" is a common phrase in gangster movies. Porn stories are rife with stock phrases, such as "glistening drop of pre-cum." In fast food joints you will often hear the stock phrase, "would you like fries with that?" or "would you like to <whatever>-size that?"
See Also: cliche, euphemism, catch phrase