My own feeling as to why people use this word: Quite rightly, at some ages, one's mind is preoccupied with procreation so it is quite natural. It is really a process where you get together with another human who has different ' Bits ' from you. The pair does the business together to bring both types of ' bits ' together . Then they completely mix up . A particular pair or two succeed in locking on and the rest die. The chosen pair or maybe few more go on to make a very desirable out come - a new Human or two.

Translating that into your feeling to when you hammer your thumb, It means " I cannot handle this I need to think of something more pleasant". OR "lets mix all my troubles up , go back to basics and start again.

I think it is far from being a swear word.

Please note this explanation can, I am sure be improved and is tentative to say the least.