to
lose no
time in beginning or starting something; to be quick or swift in one's initial
action so as not to waste any advantage one may have. This
phrase is an allusion to the beginning of a race where runners all
line up on the starting
line (called the
mark). When the
signal is
given for the runners to start, those that are quick off the
mark gain an advantage and have a
better chance of winning the race.