The spelling of the phrase "jacking off" refers to the act of male masturbation. It is spelled with the letter "j" followed by "acking off". The pronunciation of the word is /ˈdʒækɪŋ ɒf/ with the stress on the first syllable "jack". The "j" sound is pronounced as a "dʒ" which is a voiced palato-alveolar affricate. The rest of the word is pronounced as expected, with the "o" in "off" being pronounced as a short "ɒ" sound. This spelling and pronunciation are commonly used in informal and slang contexts.
The word "jacking off" is a slang expression for male masturbation, and it does not have a specific etymology like more common words. It is a product of colloquial language development and likely emerged through various linguistic processes, including euphemism and slang formation. The use of "jack" possibly derives from earlier slang terms for "self-gratification" or "stimulation", which may be related to the motion of a mechanical jack or jacking something up. However, it is important to note that slang words and their origins often lack concrete documentation, and their emergence is typically influenced by cultural context and collective usage.