I second the final part of the Phil Frost's answer.
"Sourcing/sinking" is a property of an electrical source (power supply) - it sources a current by its positive terminal and, at the same time, sinks a current by its negative terminal... the source is both sourcing and sinking. Thus looking at the source terminals, we see that a current exits its positive terminal and a current enters its negative terminal.
When connecting some elements (transistors) to the source terminals, the currents flow through them and we see that a current exits the element connected to (after) the positive terminal and a current enters the element connected to (before) the negative terminal. Then we assign the sourcing/sinking attribute to these elements... and say that the first element sources, and the second - sinks current.
Simply speaking, if the current exits a device terminal (output or input), it is sourcing; if it enters the device terminal, it is sinking. It seems strange but some inputs can source current (e.g., TTL inputs).