Noswaith is a combination of 'Nos' and 'Gwaith' which is 'Night/Evening' and 'Time' (gwaith also means work but this is a completely different meaning)

So a literal meaning of 'Noswaith' is 'Night-time' or 'Evening-time'.

Which is why the greeting 'Noswaith dda' for 'Good evening' is most appropriate ie 'Good Night-time' or 'Good Evening-time'.

Otherwise Nos (or occasionally Noson) is used for evening and night expressions.

Nos Sadwrn = Saturday Night/Evening

Gyda'r nos = In the evening

Nos Galan = New Year's Eve

Bob nos = Every night/evening

Noson Lawen = A merry evening, an expression used to describe an organised get together of performance and fun, less formal than a concert.

It is an idiom. nos can often be used for 'evening' as well as 'night'.

I agree