The Welsh word for Toxic is: Gwenwynig
The runoff from the mine was toxic.Toxic fumes filled the air after the tanker overturned.His anger became toxic.
The only case where a Welsh word would begin with ph is if it were mutated as ineu phen. That is called a spirant mutation.
Warning
That is the correct spelling of the word "toxic" (poisonous).
The word "toxic" means "able to kill" so a toxic acid is one that could kill if ingested. Vinegar contains an acid that is non-toxic in that dilution, as does the cola you may drink (but any acid in a high concentration might do harm). Strong undiluted acids like Sulphuric and Nitric acids, will kill you if you drink them.
The word 'fern' is not a Welsh word and has no meaning. The Welsh word for 'a fern' is 'rhedynen'.
The Welsh word for Excellent is ardderchog, it is also the welsh for fantastic.
The Welsh word for mum is mam.The Welsh word for dad is dad.
the welsh word for trust is 'ymddiried'
The Welsh word for 'landowner' is 'tirfeddiannwr'.
The Welsh word for 'a cave' is ogof.
The Welsh word for thunder is "taranu."
The Welsh word for 'rectangle' is petryal.
The Welsh word for Epiphany is Ystwyll.
The Welsh word for 'phoenix' is ffenics.
'You' is not a Welsh word.
The Welsh word for "Welsh" is Cymraeg :)