Learn English – The Spelling of Movie

If I understand the logic behind your question, you are asking:

Since movies is plural, why is the singular movie and not movy?

In general:

Well, just because there is a common and helpful rule that states

A noun ending in a consonant and then y makes the plural by dropping the y and adding -ies

this does not mean that you can always apply this rule backwards to any plural word ending in -ies to get the singular of that word.

That is, there is no rule that says a word whose plural form ends in -ies comes from a singular form that ends in consonant plus -y.

You can apply the rule backwards, when it was originally to a word, as in

daisies : daisy
ponies : pony

But you cannot apply the rule backwards when it was not originally applied. Thus the following does not work

monies : mony

(Note that money has two plural forms, moneys and monies.)

In Particular

And the following does not work:

movies : movy

Because the singular of movies is not movy but movie. The why of that has to do with movie coming from moving picture. Mov(ing picture).

The -ing picture was replaced by the ending -ie. I suppose it could have been replaced by y (after all, we have such nouns as gravy.

I suspect that movie(s) originally had a diminutive meaning, in which case the ending -y seems to be fine only in words whose last consonant is doubled (doggy, granny) while -ie goes with those kind of words (doggie, grannie) or...

with words whose consonants are not doubled (sweetie, birdie). I guess movvy didn't quite work, any more than movy.

A similar word to movie, now outdated, is

talkie

It has the plural talkies. The word talkie apparently does not come directly from talking pictures, but from movies. The mov- was replaced by talk-.

Note that we have 'walkie-talkie'. This conforms with -ie usage in words whose ending consonant does not double. But later on came the form 'walky-talky'. One can only guess why this variant spelling came about.