to state simply it is the reverse placement of the first letters in "licking finger"; can be used to trick people into believing you said something bad
person1: omg. that person is a fickinglinger!
person2: ..what did you say?
The use of the word "blue" to refer to sexually explicit content was first recorded in Scotland in 1824. "Blue" meant "lewd" because prostitutes dressed in blue gowns.