"On'd yw e?"
Which expressions? On'd yw e? is used as a tag (Mae e'n dal, on'd yw e? "He's tall, isn't he?") in parts of the south.
The North Wales variant, from what I remember is: On' tydy?
Where the 'd' has hardened a little.
We (Southerners) laugh (in a friendly way!) at our friend from Blaenau Ffestiniog because she's forever saying "ynditydi" "it is, isn't it?". She says it really quickly and when we pointed it out to her she didn't even realise she said it!