To gather or collect something (like courage or troops), or to assemble people for inspection or a meeting.
From Old French 'mostrer' (to show), derived from Latin 'monstrare' (to show or display), originally military terminology for gathering soldiers for inspection.
The phrase 'pass muster' comes from military drills where soldiers had to 'pass the muster'—the formal inspection—which is why it still means 'meeting standards' today.
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