pull the rug from under someone Definition

rug
noun

1. A thick heavy mat or small carpet for covering a floor.
2. A thick blanket or wrap, especially one used for travelling, or as a protective and often waterproof covering for horses.
(originally N Amer)
3. slang
A toupee or hairpiece.

Idiom: pull the rug (out) from under someone

To leave them without defence, support, etc, especially as a result of some sudden discovery, action or argument.

Etymology: 16c.