This is an archaic verb and the only forms that remain are used for reported speech in a story or report.
eg "It was wet yesterday," she said
"Roedd hi'n wlyb ddoe," meddai hi
The full forms are:
Meddwn i - I said Meddet ti - you said Meddai hi - she said Meddai fe/fo - he said Medden ni - we said Meddech chi - you said Medden nhw - they said
'Meddan nhw' is a North Wales variant, where 'a' tends to replace the 'e' or the 'y' in standard Welsh.
The song is referring to the effects of 'Brad y Llyfrau Gleision' on imposing English only education on mostly Welsh speaking Wales in the 19th century.