Calefacient refers to something that produces heat or warmth. Its antonyms, therefore, would be words that indicate coldness or coolness. Some possible antonyms for calefacient include frigid, icy, chilly, cold, and arctic. Other antonyms for this word might include words that describe a lack of heat, such as unheated, cold-blooded, or cooling. Depending on the context in which the word is being used, other antonyms could be words that indicate a lack of activity or intensity, such as stagnant, torpid, or dormant. Overall, the antonyms for calefacient are words that describe a lack of warmth or activity in some way.