If this sounds like something someone from Scotland might say when having to do a lot of boring busywork for some pernickety pest, well, yes. But it’s not an expletive. It’s a word for the tedious trivial tasks themselves, or, as a verb, for busying oneself about them. Here’s a citation:
Yet after a’, wi’ this fyke-fack an’ that fyke-fack, this thing an’ the tither thing, it cost me tippence or thretty pennies by the time I got without the port.
It can also refer to the perversity and caprice of volition that leads a person to demand such goose chases:
Yese get me na ill to be courtit For fykefacks. . . . I cuist thaim awa.
Do those quotes look… not quite like English as you know it? You’re right. They’re not. They’re Scots, which is not the same as Scottish English. If ye dinnae ken the difference, Scots is a language that resembles English in many ways but differs from it in mickle others; they split apart centuries ago. Linguists like to say “a language is a dialect with an army and a navy” (quoting Max Weinreich, who actually said “a shprakh iz a dialekt mit an armey un flot” in Yiddish, which, ironically, was a language separate from German without having an army or navy), but in the modern times we might say “a language is a dialect with its own Wikipedia.” And Scots has its own Wikipedia. (So does Yiddish.)
Scots also has its own national dictionary: DSL, Dictionary of the Scots Language, or Dictionar o the Scots Leid. That’s where I found fyke-fack, along with a lot of other words, plus those nice citations.
Where does this term fyke-fack come from? It’s formed as a common Germanic reduplication (front vowel first, back vowel after, like ding-dong, tick-tock, or see-saw) from fyke, which means ‘fidget’ and ‘fiddle about’ and a whole bunch of related senses. It in turn comes from a North Germanic source that has apparently also descended to Danish fige ‘hurry’ and Norwegian (local dialect) fika ‘be restless’ – and to Dutch fikfak ‘unnecessary fuss’.
And so, with various futzing about, we get from all that to fyke-fack, which refers to futzing about busily. It’s not actually English, but we can borrow it from a neighbour, I hope – there always seems to be a need for it.