How can I use Latex' \newcommand in Rmd?

How about using R instead:

---
title: "Untitled"
author: "SQC"
date: "21 September 2018"
output: html_document
---

```{r, include = FALSE}
short <- "AreallylongwordIhavetotypefrequently"
```

# My Test
I would like to write `r short` instead ...