General negative connotations:
* Anodd: This translates to "difficult" or "unpleasant" and can be used in a general sense to describe something that is unpleasant or undesirable.
* Cas: This means "hateful" or "detestable" and is a more intense word that implies a strong dislike.
Specifically referring to something unpleasant:
* Llwch: This translates to "dirty" or "filthy" and refers to something that is physically unpleasant.
* Drwg: This means "bad" or "evil" and can be used to describe something that is morally unpleasant.
* Gwael: This translates to "poor" or "inferior" and can be used to describe something that is unpleasant due to its low quality.
Informal slang:
* Ffwl: This means "stupid" or "silly" and can be used in a casual way to describe something that is annoying or unpleasant.
The best translation will depend on the specific situation. If you can provide more context about what you want to describe as "nasty," I can give you a more accurate translation.