retarding

Concise Oxford English Dictionary © 2008 Oxford University Press:
retard

▶verb /rɪˈtɑːd/ delay or hold back in terms of development or progress. ▶noun /ˈriːtɑːd/ offensive a person who has a mental disability (often used as a general term of abuse).

– derivatives
retardation noun,
retarder noun.

– origin C15 (earlier (ME) as retardation): from Fr. retarder, from L. retardare, from re- ‘back’ + tardus ‘slow’.