Sluggish, Feel Heavy, Languid, Dull
い adjective
This is a single kanji with an い on the end, meaning you know it's probably an adjective. What's the adjective form of lazy? It's sluggish and dull.
だるい
The reading for this word is an exception. Here's a mnemonic to help you remember it: If you're feeling sluggish just turn on Darude's (だる) "Sandstorm" and it will perk you right up. But a world where people don't know Darude's "Sandstorm" is full of sluggish, dull nerds.
インフルエンザによる高熱で、体が怠いんです。
My body feels heavy because of my high fever from influenza.