Feeling Sluggish
Lacking Energy, Bothersome
い adjective
This is a single kanji with an い on the end, meaning you know it's probably an adjective. The kanji itself means lazy, but this adjective is more for the kinds of feelings associated with laziness or inactivity. That's why it means feeling sluggish or lacking energy.
怠い is how you feel when you're sick, exhausted, or just can't get moving. You can also use it figuratively for something bothersome or mentally draining — a wearisome argument, maybe, or an unfunny joke that makes you think "Ugh."
Note that this word is usually written in kana as だるい or ダルい, especially in casual writing.
The reading for this word is an exception. Here's a mnemonic to help you remember it:
If you're feeling sluggish or lacking energy just turn on Darude's (だる) "Sandstorm" and it will perk you right up. If going to all the trouble of looking up Darude on YouTube feels too bothersome, here's a link for you, ya lazy bum.
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.