Minced oath. Esspecially if you remember that the article is mostly swallowed in many northern English dialects. "I'll go to 'f..." and the relieveing euphemistic phrase follows. Which was common practice in an age (actually a very short age, but they got to write the books) when only aristocrats and paupers could swear. The effects lasted into the age of recordings. "I can't do my bottom belly button up" managed to get two naughty words into a best selling song, and a lot of the humour of a generation or so before ours revolved around not quite saying the bad word that most (but not all) of the audience knew.