If something is skew-whiff, it is not straight. A picture hanging skew-whiff; a hat sat skew-whiff on someone’s head. It is informal British English — kitchen talk, family talk — not a word American textbooks will spend time on. In the US people more often say crooked, lopsided, or wonky. You can put it before a noun or after a verb: “a skew-whiff sign,” “it was hanging skew-whiff.” People also stretch it to plans and stories that feel off, not only to things you can see. Though actually, the second half is not whiff as in a smell or a puff of air, even though the spelling invites that idea. Hand-loom weavers in the north of England used to say skew weft when the cross-threads of the cloth were out of line. The spoken form drifted toward “wift” and then “whiff,” and the spelling followed the sound.