Because ye despise this word; rather, because ye reject this word (see 1 Samuel 8:7; 1 Samuel 15:23, 1 Samuel 15:26; 2 Kings 17:15, etc.). The "word" intended is probably the prophecy against trusting in Egypt (Isaiah 30:1-7).
And trust in oppression; or, extortion. Oppressive measures employed to obtain the rich gifts which had to be sent into Egypt (Isaiah 30:6) are probably intended. Gratz and Cheyne change the reading from 'oshek to 'ikkesh ("perverseness"); but without any necessity.
And perverseness; literally, crookedness; i.e. "tortuous policy" (Kay). And stay thereon; rather, lean or stay yourselves thereon.