[1] The most commonly inserted English expletives are adjectival: either participles (fucking, mother-fucking, freaking, blooming, bleeding, damned, wretched) or adjectives (bloody).
[2] This suggests that the rules for the placement of the expletive are not arbitrary, but instead derive from fundamental aspects of English phonology.
[3][better source needed] A more fundamental theory due to John McCarthy is based on prosody.
For example, although unbelievable and irresponsible have identical stress patterns and the first syllable of each is a separate morpheme, the preferred insertion points are different: un-fuckin'-believable, but irre-fuckin'-sponsible.
The infix cannot fall between the syllables ir and re because they form a single prosodic foot.