It’s called “thorn,” and it makes roughly the same sound as English’s “th.” (to my knowledge at least. I’m pretty sure “th” is associated with two slightly different phonemes, one voiced and one unvoiced I think, idk tho, English is stupid)

My best guess is that it originally was “fucking,” someone censored it to something like “hecking,” then someone else censored the censor back to “fucking”