You may be reminded of Dr Samuel Johnson’s famously unhelpful try at defining network in his Dictionary of 1755: “Any thing reticulated or decussated, at equal distances, with interstices between the intersections.” The moral for lexicographers is not to define a word using words that are less familiar than the one you’re defining.
The verb decussate means to intersect or cross two things to form the shape of an X. Its source is the Latin verb decussare with the same sense. This can be traced back to the ancient Roman copper coin called an as (whose name, by the way, is the origin of our ace). A coin worth 10 of them had the name decussis, a combination of as with the word for 10, decem. As the Roman symbol for 10 was X, decussis came to mean cross-shaped and the verb followed.
In everyday life we’ve never had much need for decussate, as it’s much simpler to use crosswise. Mostly, it turns up in specialist fields of study. As an adjective, neurologists use it to describe nerve fibres that cross the midline of the spinal cord or brain. For botanists, it refers to the leaves in some plants that are arranged on the stem in alternate pairs at right angles.
Decussate also describes one form of the Christian cross, the one that looks like a figure X. Another name for it is the St Andrew’s Cross, after the saint who is said to have been crucified on one that shape. A white on blue St Andrew’s Cross, popularly called the Saltire, forms the national flag of Scotland and is incorporated in the Union flag of the UK.