the mind > emotion > love > courtship or wooing > court or engage in courtship [verb (intransitive)] > sit up late as method of courtship (2)
intransitive. Of a couple: to sit up together at night as a recognized part of courtship (cf. to sit up at sit, v. phrasal verbs 1).
intransitive. English regional (north-western), U.S. regional, Caribbean, and South African. To stay up for part of the night (with a person) as…