I have a somewhat fuzzy definition. "A programming language that has more 'intent' to it."
For example, Rust. The designers had definite ideas about what they were creating, noted the trade-offs, and implemented features based on those decisions.
You can say that about lots of languages - Java, C#, Swift, or if you need to go back 10 years, Java <g />, C, or maybe even Python.
JavaScript on the other hand was cobbled together in a few weeks by some poor fellow who was under impossible deadlines. And it shows. We've all seen the "Wat" video. Programming in JavaScript is a minefield of bizarre phenomena. Function scopes as namespaces. Nutty math. It succeeds in spite of itself because, a. It's rarely used for anything truly important, and b. It runs on the most popular run-time environment on the planet.
Not that you can't do great things with JavaScript. But just about any other language would have been preferable.