What is an "idle word"

 Jesus spoke about "every idle word."

What is an "idle" word?

Thayer's Lexicon of Greek words defines "idle" as, 

 "of things from which no profit is derived, although they can and ought to be productive; as of fields, trees, gold and silver"

The second definition is specific to Matthew 12:36: "unprofitable...equivalent to pernicious. 

Pernicious: "having a harmful effect, especially in a gradual Pernicious  or subtle way".

"O generation of vipers, how can ye, being evil speak good things? for out of the abundance of the heart the mouth speaketh." 

"A good man out of the good treasure of the heart bringeth forth good things: and an evil man out of the evil treasure bringeth forth evil things." 

"But I say unto you, That every idle word that men shall speak, they shall give account thereof in the day of judgment. 

"For by thy words thou shalt be justified, and by thy words thou shalt be condemned." - Matthew 12:34-37 

"Let no corrupt communication proceed out of your mouth, but that which is good to the use of edifying, that it may minister grace unto the hearers." 

"Let all bitterness, and wrath, and anger, and clamour, and evil speaking, be put away from you, with all malice: And be ye kind one to another, tenderhearted, forgiving one another, even as God for Christ's sake hath forgiven you." - Ephesians 4:29, 31-32 

"There is that speaketh like the piercings of a sword: but the tongue of the wise is health." - Proverbs 12:18

"Pleasant words are as an honeycomb, sweet to the soul, and health to the bones." - Proverbs 16:24

"If any man among you seem to be religious, and bridleth not his tongue, but deceiveth his own heart, this man's religion is vain." - James 1:26 

"Wherefore comfort yourselves together, and edify one another, even as also ye do." - 1Thessalonians 5:11 

"Therewith bless we God, even the Father; and therewith curse we men, which are made after the similitude of God.Out of the same mouth proceedeth blessing and cursing. My brethren, these things ought not so to be." 

"Doth a fountain send forth at the same place sweet water and bitter?" Can the fig tree, my brethren, bear olive berries? either a vine, figs? so can no fountain both yield salt water and fresh." - James 3:9-12 

With Jesus, walking in His Spirit, we can do it.