it's a bit more formal as I understand it, more like ''if you please''. ''plis'' tends to be more common in speech nowadays
"os gweli di'n dda" (or the plural/formal "os gwelwch yn dda") is literally "If you see well.", using the conjugated future-or-sometimes-present form of "gweled" ("see"). I don't know how that particular phrase was adopted to mean what English usually covers with "please" or "if you please".
Thank you. I really wish they would dis-aggregate the phrases. I understand it is idiomatic, but it would still be nice to know what each word in the phrase means on its own.
"os gwelwch chi'n dda / os gwelwch yn dda" + someone you call "chi"
"os gweli di'n dda" with someone you call "ti"
Yes it means You're welcome too. I can't reply directly to your comment sorry, the Reply button has disappeared!
Off-topic, but: how do I get bold typeface when posting? I know how to get italic (**)
@Joe601911 Italic is one asterisk either side, bold is two and bold italic is three.