When he speaketh fair - For there are such hypocrites and false friends in the world.

Believe him not - Let all his professions go for nothing.

For there are seven abominations in his heart - That is, he is full of abominations.

speaketh fair or maketh his voice gracious

Psalm 12:2 They speak vanity every one with his neighbor: with flattering lips and with a double heart do they speak.

Psalm 28:3 Draw me not away with the wicked, and with the workers of iniquity, which speak peace to their neighbors...

Jeremiah 9:2-8 Oh that I had in the wilderness a lodging place of wayfaring men; that I might leave my people...

Micah 7:5 Trust you not in a friend, put you not confidence in a guide: keep the doors of your mouth from her that lies in your bosom.

believe

Jeremiah 12:6 For even your brothers, and the house of your father, even they have dealt treacherously with you; yes...

Matthew 24:23 Then if any man shall say to you, See, here is Christ, or there; believe it not.

seven

Proverbs 6:16-19 These six things does the LORD hate: yes, seven are an abomination to him...

Proverbs
Many specimens of the so-called Wisdom Literature are preserved for us in the book of Proverbs, for its contents are by no means confined to what we call proverbs. The first nine chapters constitute a continuous discourse, almost in the manner of a sermon; and of the last two chapters, ch. xxx. is largely made up of enigmas, and xxxi. is in part a description of the good housewife. All, however, are rightly subsumed under the idea of wisdom, which to the Hebrew had always moral relations. The Hebrew …
John Edgar McFadyen—Introduction to the Old Testament