To keep up is to move or progress at the same speed as the others. Keep up at the back, hard to keep up, keep up the good work.
It also means to maintain, as in keeping up a house or a payment, and to keep somebody up is to stop them going to bed. Keeping up appearances is doing everything necessary to look as though nothing is wrong.
1phrasekeep upTo move or progress at the same speed as the others.
“Walk more slowly; the little ones can't keep up.”
keep up is a matching.
Middle English kepen up means to maintain or preserve, from keep plus up.
Middle English
kepen up — to perpetuate, maintain, preserve
English, today
keep up
From Middle English kepen up (“to perpetuate, maintain, preserve”), equivalent to keep + up.
Etymology from Wiktionary, CC BY-SA