I don't have an example off the top of my head... but falling knife refers to a play where the pps keeps dropping and yet "Hope" buyers continue to average down even though there is absolutely no "hope" of it going back up....
Example...you are buying xyz stock... and you are drinking the kool aid... they put out a pr that they are going to do a reverse split as it wil lbe in the best interest of shareholders... you actually believe them and keep buying as the price continues to plummet...