I often hear "Guet Nacht!" in the evening, whereas I would expect "Gueti Nacht!" as Nacht is feminine. How so?
Endingless adjective attributes are an archaism. They are preserved in idioms, names, or composites:
- auf gut Glück (instead of: auf gutes Glück)
- unser täglich Brot (instead of: unser tägliches Brot)
- Gutmensch (instead of: guter Mensch)
- Grossstadt (instead of: grosse Stadt)
- gut Nacht (more commonly: gute Nacht)
This is exactly the same in Swiss German, even though some Swiss German adjective endigs differ from their standard German counterparts:
- uf guet Glück (instead of: uf guets Glück)
- Guetmönsch (instead of: guete Mönsch)
- guet Nacht (instead of: gueti Nacht)
Note also that some Alemannic dialects (virtually all Swiss German dialects are Alemannic) may have grammaticalized some endingless forms in the paradigm of adjective declension:
- guet Brot (Basel) vs. guets Brot (Berne)/gutes Brot (standard German)
- der schön Turm (Berne/Zurich) vs. der schöni Turm (Zurich, variant)/der schena Turm (Mulhouse)/der schöne Turm (standard German)
- d frisch Milch (Strasbourg) vs. di früschi Milch (Berne)/die frische Milch (standard German)
I do not know of any Alemannic dialect, though, that would have grammaticalized an endingless form for the strong adjective declension of the feminine singular. With other words, I do not know of any Alemannic dialect where cases like liebi Sarah (standard German: liebe Sarah) or gueti Nacht would default to be endingless. Of course, my knowledge is far from complete.