1. in the most basic way.

He was at heart a conservative man.

2. as what someone cares about the most.

Your academic adviser should have your best interests at heart.

I'm an English teacher at heart but also a writer.

• a crock of shit
• a cross has to bear
• a cross someone has to bear
• across the board
• a cross to bear
• a crowd-puller
• a cry-baby
• Act age!
• actions speak louder than words
• Actions speak louder than words.
• act out
• act the fool
• act the goat
• act up
• Act your age!
• a culture shock
• a culture vulture
• a curate's egg
• a cut above
• a cut above someone
• a cut above something
• a damp squib
• a dark horse
• a day late and a dollar short
• add fuel to the fire