What is hiutil and why is it sucking up my cpu?
It's indexing your computer drive(s).

Per the man page hiutil (help index utility) is used for creating and examining .helpindex files (Help Viewer.app indices) &amp; is used for the Help menu searches as well as for Spotlight. You can use sudo find / -name "*.helpindex" to display all of them. See here for some description from the developers site about Apple Help concepts.

Per this answer on Apple's discussion forum hiutil is indexing your hard drive to make using the Help and Spotlight features snappier. You may also have a process running called mdutil (meta data utility).  

You can kill the process - it may run for a half hour depending on system changes (note the Activity Monitor syntax for time is d:h:5m:17.85s and it is reporting accumulated time with the processor actually used - not total time since the process first started). If you want it to not run at all, you could also remove the executable file from /usr/bin/ and restart - tho I'd suggest moving it somewhere so you can restore it to that location instead of deleting it.

If the process is running in conjunction with mdutil then you can check the status of the volume being scanned with mdutil -s &lt;volumename&gt;. You may want to open Activity Monitor as root so you can see everything going on (along with - as i see in your picture -selecting "All Processes" from the View menu):

sudo "/Applications/Utilities/Activity Monitor.app/Contents/MacOS/Activity Monitor"


...double click or select and Cmd+i on a process to get more info about it and related processes. 

When this process is running, you might - depending on your OS - see an index progression bar in your spotlight window if you start to enter text (or enter "indexing").

You can adjust your System Preferences >> Spotlight::Search Results and ::Privacy panes to alter the scanning behaviour. As changes are made to actively scanned directories, the indexer is invoked.



If your computer is continually indexing and won't stop, you could add your internal drive to the "Private" list and see if it the indexing finishes.

Another option if you determine that the hdutil process is working with mdutil is to enter sudo mdutil -E / . to force Spotlight to create a new index file.