I have been unable to find the word itself, or a verb from which it might derive as a participle.
I can only suggest that it is an idiolectal or (very local) dialectal construction, on the analogy of afeard of, afeart of, building from a common exclamation of disgust which takes a wide variety of forms:
OLDER ENGLISH: foh, fah, faugh, fough, fie, fy, &c (OED 1: "An exclamation of abhorrence or disgust")
CONTEMPORARY ENGLISH: yechh [jɛx, jɛk], yik, yuk (Oxford Dictionaries: "informal expressing aversion or disgust". Also yechy, yukky, yikky, adjectives)
SCOTS: feech [fiç], feigh, feuch [fɪç, fjux] (Scots Online Dictionary: "An exclamation of disgust at a foul smell, pain, impatience or disappointment." Also feechie, adjective "Foul, dirty, disgusting, rainy, puddly")
The Scots version in particular might give rise to [fist], substituting an [s] for the un-English [ç]. Is there any Scots in your mother's background?