early 15c., "slanting, sloping, sideways; crooked, not straight or direct," originally of muscles or eyes, from Old French oblique (14c.) and directly from Latin obliquus "slanting, sidelong, indirect," which is perhaps from ob "against" (see ob-) + root of licinus "bent upward," from a PIE root meaning"to bend, be movable," the source of see limb (n.1). But De Vaan writes, "The etymology is unknown. Closest in form and meaning are līmus 'transverse' and sublīmis 'transverse from below upward', and the latter would be morphologically similar to oblīquus. Yet a root *lī- with different suffixes *-mo- and *-kwo- does not immediately make sense, and has no clear connections outside Italic."
Figurative sense of "indirect" is from early 15c. As a noun in anatomy in reference to a type of muscle the direction of whose fibers is oblique to the long axis of the body or to the long axis of the part acted, by 1838. Related: Obliquely; obliqueness.