What is the meaning of "Sefas Emes Tikon La'ad"?
Rashi: It has a base, and lasts.
R. Yonah #1: One who is careful with his words to avoid Sheker, and is meticulous to coordinate his words to Emes. His words are always heard. People believe them, and want to hearken to them. Tikon is like "Lo Tikon Atah u'Malchusecha" (Shmuel I, 20:31).
R. Yonah #2: Via telling the Emes, the hearers will always believe.
Malbim (17): Safah is light, external speech; Lashon is elevated, logical speech of Tevunah. Emes lasts by itself, without proofs. Leshon Shaker, even though it is with logical proofs of Tevunah, it lasts only a moment. Immediately the ruse is recognized. A small amount of Emes pushes off much Sheker.
What is the meaning of "v'Ad Argi'ah Leshon Shaker"?
Rashi: In a short moment it is finished off, for Sheker has no feet (base).
R. Yonah: Sheker lasts only a moment, for quickly it is revealed and known that he is a liar, and he is caught in his words. After this, people will not hearken to his words. They will assume that all of his words are Sheker. This is a liar's punishment - even if he says Emes, people do not heed him (Sanhedrin 89b). Argi'ah is an expression of Rega (a moment) with an added Aleph, like "uv'Ezro'a" (Yirmeyah 32:21), "Etz'adah" (Bamidbar 31:50), "Argi'ah" (Yirmeyah 49:19).
R. Yonah #2: One who tells Sheker is believed only temporarily. The matter will not last.
Malbim: Refer to 12:19:1:4.