If you send someone to Coventry, you ignore them or ostracise them from your group. It is form of a playground bullying and also used to punish strike-breakers.

Why Coventry?

During the English Civil Wars of the 1640s, Cromwell sent Royalist soldiers to be imprisoned in this cathedral city in Warwickshire, England. They were shunned by the locals who didn’t want them there. This is suggested as the origin of the expression.
Another theory is that it was troops who were billeted in the town that were unwelcome and ostracised.

The phrase appears in Enid Blyton’s school stories where sending a girl to Coventry is the ultimate punishment.

Another example is Charles Dodgson, who was ‘sent to Coventry’ by the Liddell family for unknown reasons, thought people speculated that it had something to do with his relationship with young Alice.