Radio amateur CW operating procedure, including abbreviations, have been handed down from commercial telegraphy operators since the time when Morse code was the only method of communication. (Seen in old western movies).
Abbreviations were used to reduce transmission time. For example the old landline operators could have been operating between railway boxes or sending a news story to an editor. Common words known to both operators would be abbreviated; for example "Good Morning" became GM, "Be seeing you" became BCNU and "Thanks" became TKS.
Letters with long Morse symbols were substituted for shorter letters, so "For" became FER" and "With" became WID. These short-cut words have been handed down throughout the years and are now used by the younger generation when sending phone text messages
Common words known to both operators would be sent by just using the first letter and substituting "X" for the remainder, so "Conditions" became CX, "Distance" became DX, "Prefix" became PX, "Transmitter" became TX, "Receiver" became RX and "Weather" became WX.
Specific services sending the same information repeatedly would substitute figures for standard phrases. For example "The Northbound train has just passed" could became "21" "Stop all trains 22" and so on. From this code radio amateurs still retain "73" to mean "Best Wishes" and "88" to mean Love and kisses.
UK Chief Morse Examiner 1990-1999