Frisian short form of names that have thanc for a first element, and of which the second element starts with an "m." The name Dankmar is a good example of that.
Meaning of this name is apparently unknown. This was the name of the last full blooded Moriori (Owenga and Otonga tribes) Tame Horomona Rehe aka Tommy Solomon (1884-1933) and Moriori elder Tame Tainui Tawarere.