Idioms :

not know sb from Adam INFORMAL to not know who someone is at all:
• Why should he help me? He doesn't know me from Adam.
Why should she lend me money? She doesn't know me from Adam.

eat humble pie(US ALSO eat crow) INFORMAL to admit that you were wrong about something
After boasting that his company could outperform the industry's best, he's been forced to eat humble pie.
get/have cold feet INFORMAL to suddenly feel that you are not brave enough to do something you planned to do:
• They later got cold feet and canceled the order.

Sb is all talk and no action SPOKEN to always be talking about what you have done or what you are going to do without ever actually doing anything

beginner's luck unusual success that you have when you start doing something new

Achilles' heel noun [C]
a weak part of something, especially of someone's character, which is easy for other people to attack:
• The team's offense is their Achilles' heel.