An Angliscization of the Irish word feic, feck is an exclamation or a very mild profanity: Ah, for fecks sake.
It is not a bowlderization of fuck, and is frequently used in normal speech.
The word came to notoriety on the TV sitcom Father Ted:
Father Ted – Feckin Mrs Doyle http://t.co/i7Vxqowx2Z
— Father Jack Hackett (@FeckinJack) June 18, 2014