Wiktionary
vb. Look within oneself to summon a particular positive quality, such as strength, energy or courage.
WordNet
Usage examples of "muster up".
I found that I could muster up something of sympathy by considering all that he was losing along with his life and wondering whether it would have been me lying there if I had come out on top five years earlier.
That's what we was doing in the basin last night, taking on stores, 'cause we're going to a big muster up in the fens, what we call a roping.
It took everything Rheela had to muster up her nerve, because she was quite certain of the response she was going to get.
Roberta asked, trying to muster up a show of righteous indignation on behalf of the rest of the human race.
She sat back on her couch and sighed, too tired to muster up the argument again.
That was the time the fleet would need to muster up and get there.