ham radio talk for "best regards" or "talk to you later", "good bye for now". Originated during the early days of radio where abbreviations shortened messages in Morse Code.
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"73 de W1AW" = "talk to you again soon, from radio station call sign W1AW"
Sheldon Coopers favourite number
He likes it because 73 is the 21st prime number with it's mirror 37, which is the 12th and it's mirror which is 21 and is the answer to 7x3.
He likes it most because 73 is a palindrome (1001001).
73 fans: "I think 73 is the best because I'm a nerd"
Leonard: "73 is the Chuck Norris of numbers"
Sheldon: "No it's not, all you get from Chuck Norris backwards is Sirron Kcuhc"
The most amazing sexual position ever! One couldn't even compare it to 69 or even the lazy dog!! It's simple enough, just get your unconscious victim... er willing partner to lie on the their stomach and place your "candy" on top of their "jiggle mountain". Relax and enjoy
Caren: Ughh... Mike left semen crusties all over my left cheek from 73ing last night
Aaron: I have some from Mike too... at least it was fun
Caren: That's for true :] way better than 69ing
Aaron: Or even the lazy dog!