'yacky da' is originally from 'iechyd da' (good health). You would normally hear it for 'cheers' (when you make a toast with a drink). The 'yacky da' can sometimes be taken in offense if it sounds like the person saying it is mocking the Welsh language - this is because it is sometimes considered a 'stereotype' phrase... "yacky da boyo" does not generally go down well in most parts of Wales! If you pronounce 'iechyd da' properly, no one is likely to take offense (yeah-CHid da - remember the 'loch' sound for the CH).
I think the phrase you are thinking of, Hazel, is 'Ych â fi', which basically means 'yuck' and can be used to describe virtually anything unpleasant.
...just don't get them confused and say 'ych â fi' when you mean 'iechyd da'!