Apparently, to have the fantods means to be jittery or nervous. It is an American expression and I have never heard it. Nobody knows where it comes from but some suggestions centre on a connection with fantasy.
Cassidy’s suggestion is that it comes from the phrase Tá fonn taodach orm, which according to him means “an impulsive frame of mind, jittery excitation, a fierce humor, a quick temper, a fuss, a fit…” He gives a number of references to Irish and Scottish Gaelic dictionaries but these are references to the words fonn and taodach (or taghdach in the modern spelling). Cassidy uses these dictionary references to prove that both these words exist in Irish. They do but this is completely beside the point. The question is, would they be found together in the phrase fonn taodach? The answer to this is a resounding no.
Fonn can be used on its own to mean inclination or desire, as in bhí mé le dul chuig an chóisir ach ní raibh fonn orm – ‘I was to go to the party but I didn’t feel like it’. Or you can attach a noun or verbal noun in the genitive to it to specify what you did or didn’t feel like doing, as in bhí fonn gáire orm (I felt like laughing) or ní raibh fonn codlata orthu (they didn’t feel like going to sleep). You don’t use fonn with adjectives, so fonn taodach is an impossible combination, as well as being fairly meaningless – what does it mean when you have an impulsive inclination?
Also, you have to take into account that fonn taodach would be pronounced fon teedakh. Why would fon teedakh become fantod rather than fonteeda? The answer is, of course, it wouldn’t. This is as stupid as everything else in this book.