Learn English – Difference between Grandma and Grandma-ma

Etymology

The word nan for grandma is a shortening of the word nana. Both of these words probably are child pronunciations of the word nanny. Etymonline describes this word as originating as a child's word for "female adult other than mother". This is why nanny is used as the word for a caretaker of children (since the 18th century) as well as a grandmother (since the early 20th century). Etymonline also notes that nanna is also a Greek word for aunt.

Grandma has similar origins. The word mama is a child's form of mother. In languages like German and English, the parents of one's parents have the grand- prefix applied to create their names. The original form of grandma was grandmama (18th century, OED). So this is simply the result of applying the grand- prefix to the child word. Then, like ma was derived as a shortening of mama, grandma evolved from grandmama.

As you can imagine, dad and pa/papa are also child words for father.

Why have multiple words

The reason there are all of these names is the same reason why most of us have nicknames for certain people or things based on childhood pronunciations. Whether that means calling your blanket a banky or your sister sessa, most of us form an emotional connection to things from this time period. Even the alternate word for stomach, tummy comes from a child pronunciation. Child language becomes a major part of family life for years, and it makes sense that some words are extended beyond the domain of early childhood development.

We retain these different forms of these words for at least two reasons: (1) each of us gives these words a differing level of significance (who wants to replace the word they use for their loved ones?) and (2) we often have different words for things that we individually use that are appropriate for different registers, e.g. kitty, cat, and feline.

Surely there should be more of a difference between the two words. If they mean the same thing surely there would be no need anymore for two words

There are a lot of near synonyms in English, so this does not follow at all. It's particularly so when you consider that act entered the English language in the late Middle English period, from French, while deed was in the language from the very beginnings of Old English. Many, but not all, seeming redundancies in English vocabulary have a similar origin story.

It is true that there are differences in nuances. The primary one would be that an act would generally be need to be relatively remarkable before we would use the term deed. One can do a particularly brave or noble or dastardly or evil deed, but one can't really do a lazy or everyday or common deed. There's nothing technically wrong with the latter, but it wouldn't be an idiomatic use.

There are similarly some differences in what verbs and prepositions are used with either. One doesn't do an act, one cannot be in the deed of anything.