An ill-defined state of irritability and distress

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
1835, jocular formation, perhaps based on fantasy .\n\nThere is an indescribable complaint, which will never allow a moment's repose to mind or body; which nothing will satisfy --which allows of no beginning, and no ending --which wheels round the mind ...

WordNet Word definitions in WordNet
n. an ill-defined state of irritability and distress

Wiktionary Word definitions in Wiktionary
n. (context chiefly dated English) (plural of fantod English) ''To have “the fantods” is to be in a state of nervousness, distress, or anxiety''.