123, in my earlier response, I confined myself to commenting on Janie's reference to the c-word and said nothing about the word you'd actually asked about.
However, I must now point out that Nightmare's answer is the correct one in that respect. The modern version of the word with an 'o' as second letter is just a variant of the much older word which had an 'a' in that position. That a-word has been in use for 1,000 years to mean a 'testicle'.
There is, believe me, no connection between pulley-blocks on old sailing ships and male genitalia. Yes, there were such blocks with that name but they had nothing whatsoever to do with the word you asked about.
Nightmare is the one who should have the three stars!