The word "builtin" implies something that is inherent or integrated into a particular system or structure. Antonyms for this word may include words such as "external" or "add-on," which imply that a feature or component is not inherent to a system but rather added on or attached. Other antonyms may include words such as "optional" or "removable," which suggest that a particular feature can be opted in or out of a system depending on the user's preference or needs. Words such as "lacking" or "absent" may also be antonyms for "builtin," suggesting that a particular system or structure does not possess a particular feature or component.