To let him/her
cook is to let them concentrate deeply on performing a certain activity. It is usually when they are doing it at a high speed or doing sometimes ingenious
1) Concentrate deeply on performing a certain activity
> The activity is being done at a high speed
> The activity requires
intellectual, physical,
mental, social, economical, financial or social skills
Friend:
Yo can you do my English
homework
Me: aight sure
Other Person: *comes in* Hey guys want some food
Me: *finishing 5000 word essay in 10 minutes*
Friends: nah let him cook