From late Middle High German schatic. Equivalent to Schatte (“shadow”) +‎ -ig. Cognate with German schattig; compare Saterland Frisian skaadich, English shady. Sound change from [t] ⟨t⟩ to [pm] ⟨tm⟩ is regular.

schatmig

1. (Uri) shady, shadowy

• Abegg, Emil, (1911) Die Mundart von Urseren (Beiträge zur Schweizerdeutschen Grammatik. IV.) [The Dialect of Urseren], Frauenfeld, Switzerland: Huber & Co., page 71.