A group of words which have a different meaning when used together from the one they would have if you took the meaning of each word separately.
Phrasal Verb:
firm something up
Meaning / Example:
1. to make something more stable or firm. > We need to firm this table up. It is very wobbly.
2. to make a monetary offer for something more appealing and attractive and therefore more "solid" and likely to be accepted. > You will have to firm the offer up with cash today, if you really want the house.