General synonyms:
* Lazy
* Idle
* Inactive
* Sluggish
* Lethargic
* Unmotivated
* Lackadaisical
* Shiftless
* Do-nothing
* Slothful
More formal synonyms:
* Inert
* Torpid
* Languid
* Supine
Figurative synonyms:
* Lazybones
* Couch potato
The best synonym will depend on the context of your writing.