A diminutive suffix is an affix that belittles, cutesy-fies, or "diminishes" whatever it is attached to. Often it is used as an endearment amongst family members or people in a very close relationship. In English, it takes several forms. Most often you'll see it as "my ____" or "my little ___." Sometimes we use a suffix from French, "-ette" or suffixes from Spanish, "-ina/-ino," or one from our own language, "-y." For example, you can use diminutive suffixes to make cute nicknames for loved ones. You could use the word for how they are related to you, like benn "husband" to make bennig "hubby." You could use their professional title, like Tân "smith" to make something like Taneg "little smith," or you can use part of their name, as long as it makes sense and is still meaningful. Take Lothuial, "Twilight Flower." You could take the "flower" part, loth, and make it Lotheg, "little flower." I wouldn't suggest it with a name like Glaewen, which means "Salve Maiden."
Other than endearments, these can also be used to insult someone. It'd be like calling someone a bad name. It's perfectly acceptable among people you're close to, but really rude with strangers or people above you in the hierarchy.
For example, you shouldn't call a Dwarf "noged." Naug, meaning "short" is already pretty insulting to a Dwarf, but noged "little short one" is a good way to get a mouthful of Dwarvish fist. Some other good insults are h�rig or brethig for bossy or proud people, meaning "little lord" or "little queen."
The more common use of the diminutive suffixes, however, is to indicate that there is only one of the things, or that this thing is smaller than other things like it. Thus, you could say that there is limmig - "just one fish" or filigod "only one bird."
The suffixes are: (they all mean the same thing, and are interchangeable).
Before adding the suffix, Make the end of the word intervocalic, and reduce the vowels. If the word ends in a -G or -C, the -G in the suffix turns into a -D.
If you add one of these suffixes onto an adjective, the adjective becomes a noun meaning "little ___ one."
Tolkien has already provided us with some pre-made diminutive familial terms:
It may be possible to use nîn "my" as a diminutive too, like we do in English. It is done in Quenya. We have examples like meldonya "my friend" and tyenya "my (familiar) you." We might have an example of it in the Sindarin title Tolkien gave to the Pater Noster, Ae Adar N�n, which means "Hail My Father," but that is hard to tell since it's a title if it could be counted as a conversational element.