sit in (for (someone))

to do someone's job or fulfill a responsibility for them.

The vice president will sit in for the president at today's meeting.

Their drummer was sick so they asked if I could sit in.

• be light years away
• be light years away from
• be light years away from something
• be like a bear with a sore head
• be like a bull in a china shop
• be like a cat on a hot tin roof
• be like a child in a sweetshop
• be like a deer caught in the headlights
• be like a dog with two tails
• be like a fish out of water
• be like a kid in a candy store
• be like a rabbit caught in the headlights
• be like a red rag to a bull
• be like a spare prick at a wedding
• be like chalk and cheese
• be like Fort Knox
• be like looking for a needle in a haystack
• be like painting the Forth Bridge
• be like rearranging the deckchairs on the Titanic
• be like ships that pass in the night
• be like taking candy from a baby
• be like talking to a brick wall
• be like two peas in a pod
• be like watching grass grow
• be like watching paint dry