I've got a beef with the word "Granny" referring only to the female grandparent, i.e., the grandmother.
Here's the heart of my contention: The partial syllable "gran" is common to both grandma and grandpa, grandmother and grandfather, so there is no reason that an "ny" attached to it should make it a feminine word. That is, of course, no reason beyond common usage, universally accepted within the English speaking world. The biggest negative against my objection is that many other words aren't governed strictly by logical considerations.
For the male grandparent we have a companion word for "Granny," which is "Gramps." There is a "p" that possibly relates to "grandPa," but where the "m" comes from, I don't know. It sounds like it was probably originally the "n" but because it's tough to say with a definite "n" sound because of the ellision into the "p" it turns out sounding m-like.
I'm not really going to make a big deal out of this, the biggest reason being that I don't think it would do much good. I am something of a contrarian, that is well established. But I'm also old enough to know that my many objections to things over the years have not typically accomplished much, making them more an exercise in futility -- if not vanity -- than something that would or could yield a lot of positive and practical good or fulfill actual personal aims of mine.