What is the meaning of "Alah Pihu Malei u'Mirmos va'Soch"?
Radak: This is the Rasha's way. When he wants to swindle others, he swears to them with a full mouth, so they will see that he swears with all intent. "Malei" is not an adjective, rather, a verb. The same applies to "u'Chevod Hashem Malei Es ha'Mishkan" (Shemos 40:34). He fills his mouth with a Shevu'ah so people will believe him, and he lies (swears falsely).
Malbim: His mouth constantly supports his words with Alah 1 , as if he fears Hashem, and he speaks deceit to swindle people.
Alah can be a Shevu'ah (he swears that he says the truth) or a curse (if this is not true, I should be cursed). We find that only a Yar'ei Shamayim should swear (refer to Devarim 6:13:4:1).
What is "va'Soch"?
Rashi: It is evil intent that is always in him.
Radak: It is deceit. There is not in his heart what he shows in his mouth.
Why does it say "Tachas Leshono Amal va'Aven"?
Radak, Malbim: When he swears with his tongue, there is sin under it. Radak - the heart is under the tongue; it is hidden. Likewise, "v'Romem Tachas Leshoni" (66:17) refers to the heart.
