In 맛있게 드세요 what is the dictionary form of the verb, 드시다 or 들다? Both of these verbs have the meaning to drink or to eat.
According to Naver Korean dictionary, '먹다' is another meaning of the word '들다'. It is an honorific form which is used most briefly.
들다 ‘먹다2(1. 음식 따위를 입을 통하여 배 속에 들여보내다)’의 높임말.
- 아침을 들다.
- 진지 드세요.
- 이 나물 반찬도 좀 들어 보세요.
들다 itself is already a respectful expression. If it is combined with '시' and converted into 드시다 or 드세요 then it becomes more respectful.