"As good as their word."
Translation:Cystal â'u gair nhw.
â'u here is the correct contraction of â + eu.
I think that the other one you have come across may be maen nhw cynddrwg â'i gilydd - 'they are as bad as each other', and in that pattern â'i gilydd is correct. This comes down to a particular construction with gilydd. It is all down to the history of the word...
(What follows is for the terminally curious...)
In days gone past cilydd/gilydd originally meant 'fellow', so ni gyda ein cilydd became 'we together with our fellows' = 'we together'. nhw gyda ei gilydd' was based on 'they (each individual one of them) together with his (each individual's) fellows' = 'they together'.
Nowadays we have lost cilydd and ended up with:
- gyda'n gilydd - we together
- gyda'ch gilydd - you together
- gyda'i gilydd - they together - the singular form ei from nhw gyda ei gilydd has stuck
And with a pattern such as cynddrwg â... + 'each other' we have:
- cynddwrg â'i gilydd - as bad as each other