Definition - What does Thumbing mean?
Thumbing refers to a method of typing that employs only thumbs. Thumbing is used on a handheld device with a built-in keyboard that is too small to type with all fingers.
May also be referred to as thumb typing.
Techopedia explains Thumbing
Thumbing is often used for text messaging or typing on a Blackberry, personal data assistant (PDA) or mobile phone. The tiny keyboard on these devices may be called a thumb keyboard.