verb intransitive To sleep too little.

What does Underslept meaning?

(intransitive) To sleep too little.

Is sleep deprived one word?

sleep-deprived (adjective)

The adjective sleep-deprived is made up of the words sleep, which is a noun here (but can also be a verb), and the adjective deprived. They are joined by a hyphen. Deprived comes from the verb to deprive, which means 'to prevent someone from having or using something'.