What is another word for deep in thought?

Here are some words you could use instead of "deep in thought":

Formal:

* Contemplative

* Pensive

* Meditative

* Reflective

* Ruminative

* Cogitative

* Introspective

Informal:

* Lost in thought

* Musing

* Preoccupied

* Absorbed

* Daydreaming

* Brooding (implies a negative or serious thought)

The best word to use depends on the specific context and tone you're trying to achieve.