Etymology
Hypercorrection of karsk. The letter "r" in Nynorsk/Bokmål-spelled words corresponds in most cases to /ʂ/ in Trøndelag dialect if it stands before s, k, t or p. Due to this change, the letter combination rk can be written as rsk (or sjk) in dialectal spelling, and vice versa (e.g. oberst can be mispronouced as obert in case of self-correction of the speaker who tries to avoid dialect in his/her speech). [1]
References
A. Dalen, J. R. Hagland, S. Hårstad, H. Rydving, O. Stemshaug (2008) Trøndersk språkhistorie: Språkforhold i ein region