Names, like fashions in clothing, drop in and out of favour. A fun time for names, I thought, came just after the "love-in" hippie era of the 1960s when babies emerged called River, Leaf, Rain, Apple, Summer and suchlike. Then there'd be a swing to biblical names and many Joshuas, Sarahs, Rebeccas, Elis, Matthews, Marks etc would fill school registers.
Friends and relatives often shorten (or occasionally lengthen) the first names of their...friends and relatives. Why this became a custom is something of a mystery; maybe part laziness (can't be bothered to enunciate the full name), part wanting to appear affectionate or "one of the in-crowd" in respect of said friend or relative. Necessity for clarity is occasionally the reason for short-form names. At around age 14, in my school class, we had three Jennifers. They agreed amongst themselves that they'd be known as #1 Jen, #2 Jenny and #3 Jennifer. Mid-term yet another Jennifer joined us - if memory serves I think she agreed to be known by initials: "JP".
How shortened names such as Richard morphing to Rick came about is easy to understand, but a change to Dick not as easy. Was it due to a bit of rhyming fun, or because a few people found pronunciation of "R" difficult (as children sometimes do)? William becoming Will or Willy/Willie is obvious enough, but...Bill/Billy? Rhyming again, as Rick/Dick... Will/Bill?
I've no idea how John became Jack. My Dad was Kenneth John but nobody ever called him anything but Jack. At this point in my keyboard scribbles I went to look for an answer to the John/Jack question, found an article on this very topic of shortened names at Mentalfloss: The Origins of 10 Nicknames. I wouldn't count shortened names as true nicknames, though I realise these do come within that definition. I'd define a real nickname as something quite apart from one's given name, whether in long or short version; something perhaps related to appearance, or talent, surname or profession (or both): such as "Nobby" once often used as nickname of anyone whose surname was Clark, because clerks in the old days wrote, using pen and ink, for hours on end, causing knuckles and finger joints to become...knobby.
My mother and grandmother used to tell me that in "the bad old days" any female who had the misfortune to work as a servant to a wealthy family was often called Mary by her employers, whether that was her name or not. Marys were sometimes called Polly too (my father's mother was an example of this), possibly via the old (southern) English habit, that still survives: putting 'L' at the end of an abbreviation in place of 'R'. I noticed in an interview between John Cleese and Eric Idle, just the other day, that John Cleese referred to Eric as "El". Terry, already an abbreviation of Terence, is sometimes further adapted to Tel. So: Mary/Mol/Molly/Polly? And Dorothy/Doll/Dolly.
