The OED says : " The word belongs to a set of words of uncertain or phonologically problematic etymology with a stem-final geminated g in Old English which is not due to West Germanic consonant gemination and therefore does not undergo assibilation.
12.
In this view, thorn clusters developed as TK > TsK > KTs and then variously in daughter languages; this has the advantage that the first change can be identified with the dental assibilation rule above, which is then broadened in application to affrication of dental stops before any stops.
