From BiblePortal Wikipedia

Webster's Dictionary [1]

(1): ( superl.) Causing no resistance to a body sliding along its surface; frictionless.

(2): ( n.) That which is smooth; the smooth part of anything.

(3): ( superl.) Evenly spread or arranged; sleek; as, smooth hair.

(4): ( superl.) Gently flowing; moving equably; not ruffled or obstructed; as, a smooth stream.

(5): ( superl.) Bland; mild; smoothing; fattering.

(6): ( superl.) Flowing or uttered without check, obstruction, or hesitation; not harsh; voluble; even; fluent.

(7): ( superl.) Having an even surface, or a surface so even that no roughness or points can be perceived by the touch; not rough; as, smooth glass; smooth porcelain.

(8): ( adv.) Smoothly.

(9): ( n.) The act of making smooth; a stroke which smooths.

(10): ( a.) To palliate; to gloze; as, to smooth over a fault.

(11): ( a.) To make smooth; to make even on the surface by any means; as, to smooth a board with a plane; to smooth cloth with an iron.

(12): ( a.) To free from obstruction; to make easy.

(13): ( a.) To free from harshness; to make flowing.

(14): ( a.) To give a smooth or calm appearance to.

(15): ( a.) To ease; to regulate.

(16): ( v. i.) To flatter; to use blandishment.

Wilson's Dictionary of Bible Types [2]

 1 Samuel 17:40 (c) We may understand from this passage that we too are to select words that may be effectively used in helping others, or in defeating the enemy, and not be careless in our statements. David knew that rough stones would not carry straight when thrown. We must choose expressions that will really go straight home to the heart.

 Isaiah 30:10 (b) Present day preaching and preachers should be influenced by this passage. Preachers should tell the truth whether or not it hurts. The smooth things mentioned are nice, sweet platitudes, which are intended to hurt no one, and to injure nobody. It is the preaching of the modernist, who fails to warn lost men of the danger that lies ahead. He gives comforting words to those who should have severe denunciation.

 Isaiah 57:6 (c) This form of idolatry is quite prevalent in our day. Probably the prophet was thinking of the smooth stones of David when he made this statement. Instead of taking the precious words of the Lord from His wonderful book, they take interesting statements of great men or women, beautiful phrases of religious leaders, and make them the law of the life. Thus the words of men supercede the words of GOD.

 Luke 3:5 (b) We read this same expression in  Isaiah 40:4. It is one of the ways in which our Lord fixes up our lives. He removes troublesome people, or troublesome relatives, or associates who are difficult to work with or live with. He takes out of our lives the things that make living hard and difficult. Sometimes He leaves the path rough, but gives His child shoes of iron and brass. Sometimes He leaves the mountains in the way, but makes the feet of His child like "hind's feet." The hind loves the rough mountains, valleys, cliffs and chasms. She does not want her way to be made smooth.

Vine's Expository Dictionary of NT Words [3]

1: Λεῖος (Strong'S #3006 — Adjective — leois — li'-os )

"smooth," occurs in  Luke 3:5 , figurative of the change in Israel from self-righteousness, pride and other forms of evil, to repentance, humility and submission. In the Sept.,  Genesis 27:11;  1—Samuel 17:40;  Proverbs 2:20;  12:13;  26:23;  Isaiah 40:4 .

 Romans 16:18

King James Dictionary [4]

Smooth a. L. mitis.

1. Having an even surface, or a surface so even that no roughness or points are perceptible to the touch not rough as smooth glass smooth porcelain. The out lines must be smooth, imperceptible to the touch. 2. To free from obstruction to make easy. Thou, Abelard, the last sad office pay, And smooth my passage to the realms of day. 3. To free from harshness to make flowing. In their motions harmony divine so smooths her charming tones.' 4. To palliate to soften as, to smooth a fault. 5. To calm to mollify to allay. Each perturbation smooth'd with outward calm. 6. To ease. The difficulty smoothed. 7. To flatter to soften with blandishments. Because I cannot flatter and look fair, smile in men's faces, smooth, deceive and coy.