Quarterly Update 24, Spring 2021
| Headword | Part of Speech | Update Type |
| cap | v | Revised Sense |
| cap corn | n | New Entry |
| capper | n | Revised Sense |
| clabberhead | adj | Revised Entry |
| cooster | n | Revised Sense |
| corduroy fence | n | New Entry |
| corp | n | Revised Entry |
| cost | v | Revised Sense |
| costive | adj | Revised Entry |
| costivity | n | New Entry |
| cradle hole | n | Revised Entry |
| cradle knoll | n | Revised Sense, New Sense |