PIE base ‘whole, well-kept’. See also kailo-, root of health, holy, and whole.
Other derivatives include safe, sage ‘herb’, solid, consolidate, solemn, solicit, and catholic ‘relating to the whole’,
It seems to be a happy coincidence that the PIE base for all the holo- words, via Greek, should have a quite different PIE base from health, holy, and whole, through Old High German and Old English.
[Pokorny *solo-, sol(e)u̯o-, pp. 979–980.]