Notice that the stem 말 changes to 마 (as in 마십시오) and to 맙 (as in 맙시다).
This is due to the fact that 말다 is a ㄹ irregular verb. In ㄹ-irregular, the stem loses ㄹ when the stem is followed by one of the following consonants:
ㄴ, ㅂ, and ㅅ. The verb 말다 is a ㄹ-irregular.
Consequently, the stem 말 loses ㄹ, as it is conjugated with the deferential imperative ending -십시오, and -십시다 (since the ending begins with ㅅ).
The other ㄹ-irregular verbs include the verbs like 살다 “live,” 알다 “know,” 길다 “long,” and so on.