This song is about a venereal disease - "The Jack" is Australian slang for Gonorrhea, which is also known as "The Clap." AC/DC lead singer Bon Scott explained the origin of the song in a 1976 interview with Sounds. Said Scott: "We were living with this houseful of ladies who were all very friendly and everyone in the band had got the jack. So we wrote this song and the first time we did it on stage they were all in the front row with no idea what was goin' to happen. When it came to repeatin' 'She's got the jack' I pointed at them one after another." Added guitarist Angus Young: "After that, wherever we did the song the girls in the audience would run to the back of the hall."
Bon Scott was known for his outrageous behavior both on and off stage. He told this story in the same Sounds interview: "One time I had the jack and this girl wanted f--kin' and she was so ugly I figured, s--t! Nobody else would have her so she wouldn't spread it. But when we'd finished she went next door to Phil (Rudd, their drummer) and gave it to him. And a few weeks later she sent him a doctor's bill for 35 dollars for the cure. Well, next time she came to a show I got her up on stage in the middle of 'The Jack' and explained how she'd got it wrong and it was me owed her the money." On mike that was.
AC/DC takes the music in their songs much more seriously than their lyrics. They would often finish songs by writing lyrics that amuse them, and this is a good example of that technique.
This was released in Australia in 1975 on AC/DC's second album, T.N.T. Their first two Australian releases were combined to form High Voltage in 1976, which was their first album released worldwide.
In concert, Bon Scott would sometimes share with the crowd a more direct set of lyrics than the one on the recording. He also did a bit where he would introduce the song by singing (to the tune of "Maria" from West Side Story):
Gonorrhea, I've just had my first dose of gonorrhea
Such poetry can be heard on AC/DC's 1978 live album If You Want Blood, You've Got It.
AC/DC played this before a crowd of 500,000 at show in Toronto in 2003. The concert, which also featured The Rolling Stones, Rush, and others, was a benefit for the city, which suffered a drop in tourism due to the spread of a rare disease called SARS (Sudden Acute Respiratory Syndrome). AC/DC had no problem singing about one disease at a benefit for another, and the fans didn't mind either.