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Related to muster in: pass muster
1. To enlist oneself in the military. All the young men my age were mustering in to join the war, but I had to remain behind because of my crippled leg.
2. To enlist someone else in the military. In this usage, a noun or pronoun can be used between "muster" and "in." The sergeant went to high schools around the country trying to muster in young men and women. They threatened to muster Daniel in if he didn't improve his grades and clean up his act.
Enlist in military service. For example, They were mustered in at Fort Dix. The antonym is muster out, meaning "to leave or be discharged from military service," as in He was mustered out and given a dishonorable discharge. [First half of 1800s]
1. To enlist someone in military service. Used chiefly in the passive: Once the men were mustered in, they got their heads shaved.
2. To enlist in military service: In the US, you can't officially muster in until you're 18 years old.