Is the word sideline a compound word?

No, "sideline" is not a compound word. It's a single word with a single meaning, although it does contain the word "line" as a part of its etymology.

Here's why:

* Compound words are formed by combining two or more words. For example, "sidewalk" is a compound word made from "side" and "walk".

* Sideline has a singular meaning, meaning "a secondary job or activity" or "to put something aside, often temporarily."

While it contains the word "line," it's not a combination of two separate words.