The Proto-Indo-European word *kar-
Word *kar-
Meaning hard
Synonyms *saiwos

Comments
Mallory J.P. and Adams D. (2006)

Other words that might be of interest
*abhro- ‘strong’, *mel- ‘strong, big’, *bélos ‘strong’, *weĝ- ‘strong’, *k̂leu- ‘hear’, *l(e)nto- ‘soft’, *meldh- ‘weak, soft’, *mel(h₁)- ‘soft’, *menkus ‘soft’