dig in
dig in {v.}, {informal}
1. To dig ditches for protection against an enemy attack.
The soldiers dug in and waited for the enemy to come.
2a. To go seriously to work; work hard.
John dug in and finished his homework very quickly.
2b. To begin eating.
Mother set the food on the table and told the children to dig in.
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Source: A Dictionary of American Idioms