To rise to a standing position from a sitting or lying position. She had to stand up to address the audience.
To remain valid or sound under scrutiny or testing. The theory stood up to rigorous testing by the scientific community.
To fail to meet someone for a planned event or date, especially without prior notice. He was disappointed when she stood him up for their dinner date.
To defend or support a particular idea or person. He stood up for his friend during the argument.