1
general::
Fail to meet expectations; lag in performance. For example, It was disappointing to see him fall down on the job. This expression transfers a literal drop to a figurative one. [Second half of 1800s]
American Heritage Idioms
2
general::
Phrase(s): fall down
to drop or topple. • The baby fell down. • Walk carefully on this ice or you will fall down.
McGrawhill's American Idioms And Phrasal Verbs