To keep awake intentionally. 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.

laborious or extra effort, as in studying late at night (cf. oil, n.1 4); usually in to burn (etc.) the midnight oil: to sit up or work (as by…

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.

intransitive or absol. to wait up: to defer going to bed in expectation of the arrival of some one.

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…