To stay awake for any work or active occupation; to pass the night in work, study, etc. Const. in, for, on or upon, to. Obsolete.

intransitive. To defer the hour for retiring to bed until late. Also: to wait up for; to stay awake through the night (or some part of it) with a…

with up. Now rare (archaic). Also †to stay upright.

To sit up (late at night). Now U.S., dialect or colloquial.

with adverbial object the night, a night (poetic). Also, to wake it.

to watch up: to sit up at night. rare.

With adverbs, as away, out. to stop on, to continue in one place or employment. to stop up, to remain ‘up’ at one's college or university; colloquial…