Using GNU sed, and assuming your file is small enough to fit in memory, you can do this:

$ sed -z 's/.*jum//' file 
ps over
the lazy
dog

Using GNU sed, and assuming your file is small enough to fit in memory, you can do this:

$ sed -z 's/.*jum//' file 
ps over
the lazy
dog

The -z tells sed to treat its input as NUL-separated, meaning that it expects a NUL (\0) to signidfy the end of each line. Since your file will have no NUL characters, this has the effect of sed treating the entire file as a single line so we can do the whole thing in a single s/// operation.

Note that this will find the last jum in the file and delete everything before that. If there are other occurrences of jum earlier, those will be removed.