nice is a command on its own, it isn't an option to the
nice - run a program with modified scheduling priority
nice [OPTION] [COMMAND [ARG]...]
Run COMMAND with an adjusted niceness, which affects process scheduling.
With no COMMAND, print the current niceness. Nicenesses range from -20
(most favorable scheduling) to 19 (least favorable).
add integer N to the niceness (default 10)
So for your purposes, you would use this:
nice -n -20 cp -r dir1 /d/