Sadie:

~Greg has the market cornered on pet names. When he’s talking to the boys, every other word out of his mouth is a pet name…and not just the boys; Mycroft too. It always varies, but Sherlock is usually ‘Muffin’, Jawn is ‘Monkey’, and Mycroft is ‘Peach’ or ‘Peaches’.
~Mycroft knows exactly why he’s ‘Peach/es’…and it never fails to put a little colour in his cheeks (so to speak). Greg lives for moments like that and no matter how often he calls the eldest Holmes brother by that name (which is daily), it always catches Mycroft by surprise and tickles Greg shitless.
~It doesn’t take Mycroft by surprise all that much anymore, but he loves the cheeky look on Greg’s face whenever he thinks he’s gotten one over on either of the brothers, so he’ll always act surprised anyway.
~Each man has their own set of pet names they use. Greg is the gushy one, with a full catalogue of names such as ‘muffin’, ‘monkey’, ‘cuddle bug’, ‘cupcake’, ‘squishy’, ‘babycakes’, ‘piddle monster’, ‘gremlin’, ‘nummy-nummers’, etc. (It’s almost enough to make them all gag…almost, but not quite).
~Mycroft is slightly more reserved, with endearments such as ‘dear’, ‘darling’, ‘lovely’, ‘dearest’, ‘pet’, and ‘little one’.
~Sherlock is very similar to his brother with using pet names, but Greg’s gushy,mushy style has leaked over. ‘Honey’ is a favorite of his to use, although sometimes it turns into ‘Honey biscuit’ with a slight twang (John blames it on too many youtube videos, but it makes him laugh). But his favorite go-to name for Jawn is usually ‘Little Man’. Depending on the day, using that name either gets him a glare, a kick in the shins, or a lapfull of happy, blushy hobbit.
~ ‘Hobbit’ or ‘Hobbit-son’ is another set of names that might get him a kick in the shins. Or, earns himself a louder, much less affectionate pet name: “You fucking cock.”
~John had a harder time with pet names, in the beginning. He never really heard them growing up, so they were foreign to his ears, as well as his tongue. Being around Greg did help quite a bit, but it was a rocky start. He would use the names he’d heard often enough, but slowly began to come up with his own.
~One of John’s nicknames for Sherlock is ‘Bumble’. But, seeing as Bumble is also the name for Sherlock’s favorite stuffed friend, this has caused some mild confusion:
“Does Bumble need a nappy change?”
“…Bum’bah doesn’d wear na’bbies.”
~While John will absolutely tear someone in half (not literally, mind you…but close enough) for calling him any variation of the word ‘short’, he’s taken to using those names to make Sherlock feel tinier (and to see his sweet little face blush, but that’s an entirely separate story):
“Munchkin.”
“Nooooooooooooo.”
“Short stuff.”
“Nooooooooooooooooooo.”
“Daddy’s Precious Little Pixie-Faerie.”
*blushes and covers eyes even thought John can still see him smiling* “NOOOOOOOOOOOOOOOO!”
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More headcanons: Naptime Edition